I was under the impression that your fact of biology was false.

Specifically that the brain-muscle mapping isn't preprogrammed and the brain learns to control what is there.

For example people with fully formed extra digits can just have a fully functional extra finger.

This does get baked in pretty well but the adult brain can re-learn after a catastrophic injury so it's not hard to believe you could figure out how to add virtual appendages or entirely novel "extremities".