How do I set the default RSS reader in Safari 6 on Mountain Lion
Further research yields this solution to the problem:

http://www.rubicode.com/Software/RCDefaultApp/

RC Default App is a (free) preference pane install that allows you to easily change what types of media are opened by which application.  Although it has not been updated in a while it still works on Mountain Lion.

I think this will solve your problem.  I've tested it on my own Mac.