Firstly, I assume you're talking about unraring by hand in Newsbin. Newsbin doesn't use any external programs to unrar normally so, what we're talking about only applies to manual unrar from the files and/or download list
In theory (and there are some online examples) a simple registry edit in wine would change the path to the rar executable however the rar executable in Newsbin I believe is compiled into the program and does not use a registry key.
Can you make a script or cmd file that executes the winepath and runs the RAR program then register this script as the handler for RAR files? When doing a manual unrar or double-clicking something in the files list. Newsbin just asks windows to invoke whatever registered program is for the file type. So, it seems like you don't have to make any changes to Newsbin to make this happen.
If you're trying to replace the unrar code Newsbin uses for normal unraring, it's pretty much impossible. If you're trying to make it unrar to some path outside of Wine, can't you link a normal linux folder into the Wine folder tree and then make that the unrar path?