If you don't have clearance issues with getting the Xtracycle around corners, at only +8lbs its not hard to take up stairs and would be much more universally practical than a chariot (mind you, the chariot would be so cool. Needs some golden lions painted on the front and such!)
The biggest problem I can think of with "chariots", is that the old ones used to have large wheels and wide stances, with someone standing straight up in one they need to have a very stable footprint or they tip. That would mean you'd need to have quite a wide profile, and it would be difficult to make the chariot small.
Oh, but yeah, some of the Zoo bomb pictures i've seen have chariots in them, if you were wanting to see some ideas.