I like the "hidden advantage" stunt idea, but it seems a little lackluster. This is going to break out of the "standard stunt" template even more, but for a once/scenario stunt (instead of once/session) I think it needs to.
It looks like the idea is that at some point when you have extra narrative power (Fate points) to blow, you can basically bank it for a rainy day, with interest.
- Assumption: When a PC is cashing in on this stunt, he's out of FP.
- Assumption: Based on the heist concept, this should be a 1/scenario instead of 1/session stunt. Your answer's example seems to agree.
- Assumption: Again from the heist conceit, the stunt should be used early and cashed in on as late as possible in the story.
- Assumption: The cash-in should be expected to successfully reverse fortune in most instances.
Now, free invocations are lovely and can stack in ways regular invocations can't, but Fate points are much more versatile: They can become invokes OR story details OR compels OR buyout from compels...
Luck In The Bank: Once per scenario, when you are not in a challenge, contest, or conflict, you can spend a fate point to end the scene and create a "hidden" aspect. At any later time you may "reveal" this aspect by determining what it is. You get one free invocation on the aspect when it is revealed, and gain one Fate point for every session since the one in which you created it. (If you reveal the aspect in the same session you created it, you instead get 1 FP for every scene since the one in which you created it.)
When you reveal the hidden aspect, it should be related to the cutaway scene, but you can attach it to a character or environment that wasn't present at the time if you can reasonably justify it.
I feel okay with making this stunt more powerful than most, because it's 1/scenario and should make up for basically having a dud stunt slot in every other scene for the entire scenario. The parenthetical FP-per-scene version is for one-shot games; if your scenario lasts more than 1 session, I strongly suggest that you encourage players to wait at least until the next session before cashing in. It's more dramatic that way.