The curious thing about Michael Levin's work is that they figured out "top level abstractions" in bioelectricity that tell the body to figure out everything, including the proper wiring. The have grown eyes in the wrong places that figure out how to connect to the brain, and even two heads.

These guys are decoding the actual bioelectrical code that governs morphogenesis, in a way that you don't need to micromanage anything, you just say "grow an eye" and the cells figure out the rest.

Plus extensive musculature at the new shoulder joint and reinforced bones for the new muscle attachments. I won't be first in line.

It doesn't work like that at all. Your brain's already plastic enough to be able to (quickly) accept new sensory input if you were given a new sense via prosthetic. Being able to twiddle your third thumb after it grows would be automatic.

I get the sense that if it's connected, your brain will figure out how to make it work.