I personally haven't see that done, but then, I haven't seen many weddings with flower girls and ring bearers either...
I think the wagon idea could be pulled of quite tastefully if it were decorated white and maybe with the bride's colors or flowers to match the decorations on it. I also think it could be really cute to have him holding the flower girls hand (his aunt, right?) He's still young enough that you will need to have him restrained in some fashion the entire time. I know MY kids like to run all over the place at that age. If the flower girl walked him down, he'd have to be left in the care of a groomsman (probably being held) so he doesn't run off. If it's going to be a long ceremony (anything over 5 minutes) the wagon may be the way to go simply because he will be contained somewhat (and you could probably put some quite activities in there to keep him preoccupied. Let's face it... He's not got a long attention span yet). Also, I'd probably make arrangements for the rings to be put on another persons person till it's time for him to give them over. My 18 month yr old would be likely to eat them still, and I don't think the bride or groom would like to wait for them to pass to get them. Just something to think about.
It sounds fun. Good luck with whatever ends up happening. He won't remember it, but it will be something fun for him to look at pictures of when he's older...