I'm with Oceanbaby -- we let the sleeping dog lie! And it's not because of the bedtime thing (thankfully, we can usually get him to sleep at his normal time of 8:00ish even if he's had a really late nap). It's because he is SO dang grumpy and tyrannical if he's woken up. Actually, he's miserable even if he gets to sleep (what is it about naps after 3:00 making kids cranky?), but he's WAY worse if he's woken up.
As for trying to keep them awake if they've missed their normal naptime and it's getting late in the afternoon, we try to do this and usually succeed if it happens to be a leisurely weekend and we can go off to the park or do some other fun, active thing with him until it's closer to bedtime, but if that's impossible for some reason (and there are plenty in our lives, and we only have 1 child!), we let him fall asleep and try to nap ourselves.
Good luck to you!