How do you treat a large evolving hematoma that’s become tissue after a Mommy Makeover?

I am sorry to hear about the complication you have experienced.  It is possible that the collection of blood has solidified;  if so, it may take weeks to liquefy and dissipate. There may have been some misunderstanding;  hematomas do not "become tissue". Close follow-up with your plastic surgeon will be important. Again, options include allowing for the flood collection to liquefy (and be able to be aspirated or allow for your body to absorb).    See if your plastic surgeon recommends the judicious use of warm compresses (if so, care must be taken to avoid burning the skin which may be insensate after tummy tuck surgery).  Surgical drainage is an option as well. Best wishes.

and if you were not aspirated regularly, a pseudobursa may have occurred and this will require further intervention and removal of that bursa.  If you have a fluid wave, you still have fluid.  Hematomas to not turn into tissue per se so you are suspect for still having fluid with that bulge you have.  You can determine what is there with an aggressive blind lipo cannula aspiration (better than any needle) or an ultrasound to ensure that you do not have anymore fluid.