Garthanos
Arcadian Knight
I considered making it analogous to the -5/+10 feat a maneuver can always be done at a penalty if you arent spending that exertion/dice.I have been thinking that a free manoeuvre on a crit would be a nice little bonus. But I have no idea why it would be random; just let the player choose.
I think it would enhance the fiction and verisimilitude. The fighter indeed always is able to do this stuff, it just is hard and they need to be lucky. The "power pool" is just to represent the focus/grit etc that allows them to do it reliably at will.
Having a critical act as a stand in works too.
One could combine this with the Martial Trick idea : Kind of like saying the first time you try a trick on a target or group of targets its has straight odds but subsequent attempts are at -5.