This seems like a great idea. Build some sort of universal bridge for media keys that any compatible app can hook into? Or would the OS X sandbox prevent that, too?

It is a great idea and should be possible so long as Apple is willing to approve an sandboxed app that accepts incoming connections (listens) on a port against the local interface.

The bridge could be the listener, and the sandboxed apps could open a connection to it. This is probably preferable design anyway.

The apps should request subscription to a set of keypress events, the bridge should get approval from the user (maybe only if the app requests keys other than F keys and media keys?). The bridge should be discoverable by local bonjour rather than running on a known port.