Can't say I've done anything quite like that, but I tend to throw in little 'mini-games' in the stuff I run.
For example, in my D&D game (3.5 all da wayyyy yeah), at about the 3/4 mark to the end, the PC's must pass the test of the God of Strength and War. As Gods are Gods and can therefore do Godly things, they will all be given the opportunity to write up an 17th level character (they'll be about 10th or so by the time they get there) who they will 'become' for a brief, RTS-inspired wargame kind of affair.
you know, the 'ok so now you're a bad-ass. There's a big army out to stomp you and your families into the ground. What are you gonna do about it?' kind of thing. They can go out and acquire and army or attempt to single-handedly (rather easy for a 17th level caster) beat the opposing army. It's not really something they can fail (unless they do something monumentally stupid), but I figure giving them the reigns of a hilariously high-powered character they can build however they want for a 'minigame' will be a fun thing.