From looking at the UI in the System Preferences App, it looks like the application firewall rules in OSX's firewall are "all or nothing" -- "Allow all" or "Block all" effectively. I'm under the impression that this UI is ultimately editing
com.apple.alf.plist behind the scenes.
Is there a way, perhaps by editing that plist, to get more granular rules? (like "only allow connections from this range of IP addresses, etc.")