There are like 3 bark textures included with paintball2. I'm not sure if any of them will work, but you might try them out.
The community texture package is sitting at 5 right now, with hopes of doubling that.
Trees [...] with models, [...] won't have any collision detection.
As you probably noticed, I'm really interested in the development of this game, and I noticed a while ago you did an analysis of pros and cons to a new map format, which coincided with the Quake 3 source releasal. Honestly, do you think we'll ever have solid models?
Personally, I really believe in a paintball game like this it's probably one of the best things we could have in maps.