is there a feat for multiclassed monks that increases your stunning fist attempts per day? I know there used to be a similar thing in 3.5, but i'm not sure if pathfinder ever made one.
Well, the Mantis Style tree gives an extra use with each feat,bit othe rise you can only get the normal. +1/4 level uses.
Fighter with Advanced Weapon Training (Abundant Tactics) can add his Weapon Training bonus to the number of times per day he can use feats with limited uses per day.
Technically, you can pick that up with a feat.
Ascetic Form will allow what you're asking.