Having spent years fighting professionally in the USA and Asia, I can tell you first-hand that you'd be surprised at how dramatic of a difference a little change under the glove can make. The rules are in place for a reason.
Granted, in MMA, the gloves are much lighter and the difference is more pronounced, but, even in boxing, guys will do certain things like soak their knuckles in ice-water before a fight which actually can make a difference in the impact of the punches.
While some plaster under the padding won't do something insanely dramatic like turn a jab into a KO punch, it certainly would begin to show its advantages as the fight goes on and the blows begin to accumulate. Being hit over and over and over by a plaster-reinforced hand will certainly result in a lot more damage than it would otherwise.