You might want to try this stuff:
http://www.rangerpaint.com/
I bought a small can and experimented with the application, but never got around to testing a whole tire on the street. It was pretty durable stuff and could only be removed with a lot of lacquer thinner. If I ever buy some Billboards for my spare set of rims, I will give it a full test (this won't happen for a long time, so don't hold your breath). Anyways, it's an option and the "kids" that stumbled on this stuff may actually have found a good solution!
Most of the paint pens are not very durable and you will get cracking after a little road time. Some have used titanium based
oil paints with success.
-Dean