Ok, thanks for the clarification. I've done some work with theunarchiver, mainly to clarify the licensing. I've also added some C bindings that can be used to work with theunarchiver in a C project.
All this work is in the "mcitadel-theunarchiver" clone in googlecode. Note that I removed the official unrar sources in this clone, so you'll have to cherry pick commits if you want some of my changes.
Is there anyone doing a C port of the unarchiver? The project I'm using this for is XBMC. I would be interested in a C port, particularly to remove the need for GNUstep or some other objective c framework. I would be glad to help out with a c port. XBMC is already huge as it is as far as the amount of libraries it uses plus code written for it.