You CAN use Might to break out of a grapple.
For any action that would be affected by the block, if that action's roll exceeds the value of the block, the block is broken.
I know. I was responding to Sanctaphrax when he said, "Anyway, I don't think there's any such thing as a Might action specifically to break a grapple."
No stunt is needed.
The only opportunity, by RAW, for such a stunt to 'allow' reverse grapples is by obviating the need for an aspect to be tagged/invoked representing the opportunity to engage in the grapple. And given the likely ease with which a declaration could be made to satisfy that requirement, the usefulness of such a stunt is highly questionable.
Not necessarily. "I'm being grappled," the example given before, isn't really an aspect that justifies you being able to reverse it on its own. It would take a lot of the bite out of the initial grappler's action and is, frankly, unrealistic unless you're a skilled grappler, which would be reflected in the stunt.
If someone tried that kind of declaration, I don't think a GM should allow that aspect alone to justify initiating a grapple. It'd be like, "So, your declaration is establishing that, as stated, he's got your arm pinned behind your back, and he's got his arm around your windpipe, and somehow this gives
you enough of an advantage to put him in a grapple?"
If anything, without the stunt I'd rule you'd need to actually maneuver to create an aspect on the opponent giving you the opening to reverse the grapple on a later turn, which would mean you're breaking the grapple anyway by succeeding at the maneuvering roll. You could
maybe guess/declare one of the target's aspects, like, "I roll Fists to declare he has SLOPPY TECHNIQUE," but otherwise, just declaring, in essence, that you're at a disadvantage shouldn't cut it.