Yes, you can still practice challenging asana if you have Diastasis Recti!
I started Ashtanga 23 years ago, four months after the birth of my third child, my son. He was a big baby, as all of my babies were, close to 9lbs, with a big head, 95th percentile! I was in perhaps the worst shape of my life when I started the practice. But, I loved it, and became a fairly consistent practitioner, considering that I had three children under six when I started, beginning with one day a week in the studio (it was all I could afford and had time for, with three little ones!) and working up to six days of practice, in about three years time.
Back then, postpartum care was not really focused on the mother's overall return to fitness (is it any better today?) and so I really didn't know that I had diastasis recti (DR). No one told me; I only knew that the tissue between my rectus abdominus was split, mostly around the navel, three-four inches from top to bottom, and fairly deep; I could shove my middle finger into the split past the first knuckle! I knew this was an issue, but there was little guidance as to how to "fix" it back then (no youtube videos 23 years ago to give us tips and tricks for "gaining a firm stomach"! ) My doc told me to "do crunches," and that was that. I'd always had a bit of a belly, and so, it was just something that I thought was normal, too, part of having babies.
I'd been told by my first teacher that uddhiyana bandha (UB) should only be engaged "below the navel," so as to not impede the breath, and like some things that you hear when you first start doing an asana practice, you do it without thinking "Does this apply to me and to my body? Is it detrimental or beneficial?" I don't believe in questioning everything, but the linear nature of Ashtanga, along with a tradition which fostered a hierarchy of domination - "never question the teacher!" - made me do as I was told! I have since shifted my perspective on this adage of Ashtanga, realizing that while the teacher may know more than me about the practice, they do not know my body better than me. (And as a teacher, I bear this truth in mind, too, with my own students. A sensible, healthy respect for the teacher must be mirrored by an equal level of respect for the student. That is, the respect must run both ways! This approach to the dynamic of learning can help dismantle hierarchies of control, that as we well know, all too often lead to abusive or harmful teacher/student relationships.)
Unfortunately, the result of only engaging UB below the navel was that I would not engage muscles where I actually needed to to help resolve and heal the DR, which specifically WAS at my navel center, where the split was the worst. Still, I would work hard in navasana to ensure that my abs were engaged, with my navel pulled in, too, chest out, in order to rebuild my core and strive not do the posture entirely with my hip flexors (which is incorrect.) This helped me gain strength and not worsen the DR, along with strong engagement of UB in down dog, too. I was able to bring the UB up to my navel and still breath fully and deeply, because that softened and split tissue needed to be toned, and DR was not something anyone developing the practice (generations of Brahmin men) had ever considered.
So, the DR improved through my Ashtanga practice, but I still had a bit of a split at the navel. And we have to remember, now, this was perhaps 18-20 years ago, and literally NO ONE talked about DR two decades ago, so there were no resources within the yoga world addressing this issue. I was on my own.
So, how did I heal it? Well, in the third year of my practice, I was taught uddhiyana kriya (UK), as a means of cleansing (since that is what kriyas are for!) but not as a means for fixing diastasis recti. So, earnest student that I was, I began to do UK on a daily basis because a clean digestive tract meant lighter practice! After a few weeks, however, I began to notice that the DR was improving, even healing: the seam between the muscles was being "sewn" up from the inside through this kriya, which actively engages the entire abdominal musculature quite vigorously on an exhale retention, drawing the navel back to the spine and holding it in as part of the kriya process. I realized UK was vital for my healing of the DR, and began to do it in earnest ad hoc whenever I had a chance, which was a couple of times a day, on an empty stomach.
As well, flying in the face of what teachers had taught me, I began to bring the engagement of uddhiyana bandhu up to my navel during my practice, and not simply below it as I'd been told to do, so that the DR could be repaired during and throughout my whole practice, too, and so that I would not worsen it! I did this intuitively, again, no one talked about DR back then. This adaptation of UB really helped accelerate the healing, and the seam was almost completely repaired after about 18 months of daily practice with stronger, longer UB, and daily UK.
About a year after I'd healed, with the seam almost fully repaired (around year six of my practice life) I'd began working on second series, and was attending a second series workshop with a senior teacher. During the led class, the assistant teacher lifted me off the floor* when I was in dhanurasana. It shocked me to be lifted off the floor by my ankles, and I lost my breath for a moment....and I felt my DS split again at the navel, although I wasn't quite sure what happened. I just knew it felt weird, like tearing.
I developed an umbilical hernia less than a week later, about 1/2" long. Instead of getting surgery, I decided to climb the mountain of repairing *this* tear with taping, bodywork, oiling of the area to prevent scar tissue, and lots more daily work with UB and UK. It took me about 18 months to repair the hernia.
This was about a decade or so ago, and it is fine now, or as fine as it will ever be. I no longer have a hernia and the DR is negligible, mostly a shallow area around my navel about an inch in diameter, that indicates a thinner abdominal wall. I still do UB up to my navel area to maintain its integrity, and I do UK a few times a week, but for strictly cleansing nowadays.
Still, I am very careful in all my backbends; I don't push it, no grabbing ankles for me in this lifetime (not that I had any desire to do so) and sometimes I've strapped a yoga belt around myself at the navel to ensure that there is no further damage when I do deeper backbends like kapotasana. I also notice things shift around my core/abdominal strength depending upon where I am in my cycle, and adjust my practice accordingly.
One thing we must consider as both teachers and practitioners: the practice is highly adaptable, and can help heal us, as well as hurt us if we are doing it incorrectly. Incorrect means many things, and often it's flung at us by folks who want to legitimize their own actions by insisting everyone does as they do. I contend, we must be willing to look outside of the box, and remember that Ashtanga Yoga is not "one size fits all" but rather, a sadhana that must take into account the individual student and their needs vs. the needs of a culture that has a tendency to reward conformity, and decry iconoclasm and heresy.
*(This was a showboat move on the part of the assistant, and unnecessary and dangerous assist, really. I have often found assistant teachers to be more prone to doing less mindful assists that have the potential to injure students. Perhaps this is because they are nervous about being seen as worthy for such an honor? For those who are assisting a senior teacher, or for anyone teaching, really: consider that less is more, and that there is no need to prove you are worthy by doing fancy adjustments, especially on students you have never met!)