Until our last trip over Easter, I'd never heard that you were allowed to use FPs after the fact, but I read it somewhere...so we tried it. We were very polite to the CMs, and usually said something like, "We know we're late for our FP time, but may we please ride anyway?" We were not turned away for any rides, and the CMs all said not to worry about it, that as long as we weren't early, they could let us on. It certainly made for a less stressful tour of the park, knowing that if we were a few hours late it was okay. However, if they had turned us away, that would have been okay too, since technically we WERE late. I think, though, as long as it's not super busy (although for us it was Easter Sunday!) and you're nice about it, CMs will generally let you on.