Any code that I wrote from scratch I'm happy to re-license under BSD. That would include the RAR code, but I have to warn you: It might be a lot more work than you think. RAR is a real mess of a file format, and the implementation in The Unarchiver may not be easy to follow, no matter how hard I've tried to make it such. However, having a portable and open implementation of RAR would be a very good thing, so I think it is worth a shot.
(Also, it has at least one known bug that I need to get around to fixing.)