Seems like a 'no' but figured i'd double check. If I have a power bonus to all 'defenses', would a power bonus to just AC stack on top of it?
No. If you have a power bonus to all defenses, that means you have four separate power bonuses: one each to your AC, Will, Fortitude and Reflex. Any other power bonus to your AC does not stack with the one already there, and only the greatest of them would apply.
The fact the power bonus applies to multiple things doesn't really matter, since it's just a single power bonus to each of those things.
Note that if you have a +3 power bonus to all defences, and a +5 power bonus to AC (which is better), you'd have a +5 power bonus to AC and still have a +3 power bonus to Will, Fortitude and Reflex. If one bonus overrides another, it'll only override the specific bits it applies to.