This feels/seems familiar... I would swear I've seen the choose-your-ending done before on TV. I could be wrong, though.
But it should be easy to pull off, as they obviously have all of the endings filmed and they can probably have the files queued up for transmission and just need to flip the right switch for that one to go out live.
I would guess/assume the eventual DVD set would give us all the different endings.
I don't really like these kinds of gimmicks, though, because it means the rest of the episode has to be written in a way that leaves it all open until that last segment... which means the writers get to write a show that doesn't really logically follow any particular path. It's billed as a "get involved fans!" moment, but it is really a cop-out kind of moment from a writing perspective I always feel.
Think football, when they say... if you have more than one QB then you have no QB. IF you have multiple endings, then you have no ending.