If its plaster of paris which is casting plaster with no added cemicals for flexability and glue style hardening then its a Stupid ass thing to do for no real massive gain.

We use casting plaster at work every day it isnt really that strong would be good for a round or maybe two but would be dust and then with the sweat would get softer still.
If it was still in lumps after a fight then there was alot in there and it was thick to start with and basted between layers of guage like laying up resin and cloth. Plaster on its own has no real strength same as resin.
If its in there in visable quantities after that many hits ,they ment it to be there. Its no mistake like a "whoops we used some powder sorry we thought it was talcum powder",wont cut it.