While I don't know of any products like this, another alternative is to use adhesive vinyl. It's great for automotive use. I have it on a truck that I take to Bears games... a big 3 foot wide "C" symbol on each side, and a smaller 1 foot by 1 foot "growling bear" logo on the back door. It's survived the harsh Chicago winter with no problem.
A Chicago Tribune article last week talked about how custom shops are forgoing traditional airbrushing and pinstriping in favor of vinyl.
Any local sign shop can make this stuff. It's even easier if you have a program like Adobe Illustrator or CorelDRAW on your computer- you can draw out your callsign, etc. and have it made. You can install it yourself or have the shop do it.
A good friend of mine works at a local sign shop (http://www.omnisigns.biz
) and did the work for me as well as a logo for his own car: http://home.attbi.com/~n9oa/pics/van-back-callsign.jpg
73, Chris KC9L