" I was in costume and am a nice person, so I probably would have been 'accepted' should I have attempted to gain access;"
I don't think my perspective on this is unique:
We had an art car this year (the furry saab with wings), and were happy to give rides to as many people as possible. We loved interacting with people who were interested in our project. And we met a lot of great new people and had a lot of great discussions with our passengers throughout the week.
Some suggestions about keeping art car drivers happy:
1) Don't just jump in/on. Do the operators the courtesy of asking nicely for a ride. If there is space, they will most likely say yes. (this probably doesn't apply as much to the larger buses.)
2) Be willing to go where the car is headed. Art cars aren't a private taxi fleet there to do your specific bidding. And they are not required to bring you back and drop you off at the same place you got on.
3) Be respectful of the vehicle. Most of them took an enormous amount of time to create, in addition to a fair monetary investment in supplies. If something looks fragile, ASK before sitting or standing on it. If your feet are coated in mud, hang them over the side. And don't even think about leaving your trash/moop behind when you get out of the vehicle.