The only one I've been to is A-kon, in the D/FW 
area (Tx). It's not particularly good to go to 
for cels, though.

I do know that last year, someone had *some* sort 
of anime-themed car, but since I heard about it 
secondhand, and hadn't seen the anime it was 
based on, I can't remember. ^_^;; So, yes, 
there's precedent for showing off anime-themed 
cars... but it's more of a "Hey, did you see that 
in the parking lot?" rather than "Go *here* to 
see this special car!"

Unless you're a special guest-- ie, one that the 
con invited to come-- I don't think they make 
arrangements. If you wanted to show off your car, 
you'd probably come as a regular con-goer. I 
don't know how other conventions are, but A-kon 
is always in a hotel, so you just ask for the 
special convention rate. But I'm assuming that 
most big conventions take place in a hotel.

I guess I'm not much help, but I'm sure there are 
others who can give you specifics on other 
conventions. 

Good luck,
-Cres