Meanwhile, I was thinking more along the lines of FMA's law of equivalent exchange, but since the way equivalent exchange is handled is kinda hand-wavy, I tried to find a way to make it more realistic, while trying to limit the power's OP potential (hence, the random body part stipulation as initially stated).
TBH, I don't watch One Piece, but had I done so, I would have given you a nod by saying you need to be concentrating to use the power (and would backfire by randomizing the target of the power if you lost concentration).