I've only been to cons on either coast, so I 
can't be much help with cons like Acen or A-kon. 
The east coast cons do decently for cels, though. 
You can always count on Curt from Asylum to be 
there. And the one west coast con I've been to 
had Anime Link (who had the most drop-dead 
gorgeous Tennimon cel I've ever seen. sigh).

Now, from the other perspective, as a member of 
the Guest Relations committee for one smaller 
con, I can pretty much assume that you won't get 
guest status for bringing an anime-themed car to 
a con. While anime cons do occasionally have fan 
guests of honour, it's going to be someone 
particularly well-known in the fandom 
(cosplayers, long-term fans who've somehow 
contributed positively to the industry, etc.). 
While it can't hurt to ask, I don't think a con's 
going to confer that status and offer you a room 
for your efforts.

A.