RE: BYYB, remember in the beta when you could "BXR" the pistol, and people said you could "double melee" simply because they'd see the melee animation twice, faster than if you were actually pressing B two times in a row?
In Halo 3, the animation locks at a point that dictates impact, which is to say, once a frame hits a certain point, it can't be canceled to swap weapons, fire, whatever. That point in Halo 2 was much more flexible, allowing you to do all kinds of melee related chicanery, same with Halo: CE, however in Halo 3, that lock is pretty late in a given sequence, so while you may be able to make it look like you are double meleeing, it's pretty likely that the frame of the animation that dictates "impact" hasn't occured yet, because the way the system was engineered, it was designed to cut off the ability to "cancel" out of an animation when a melee does damage to another player.
Simply and more coherently put: While a double melee may look like it's occuring, I am pretty skeptical and doubtful that it would hit a player twice, and have seen exactly zero evidence to support it against another player. It's basically the world's longest and most convoluted melee.
That's not to say I don't think stuff will surface, millions of players testing and trying to break stuff and find advantages are definitely going to find stuff our Testing Army didn't. But as of right now, I still haven't seen anything.