Comment 47 for bug 704231

raof, yes Unity does cause Compiz to add passive grabs on Super+AnyKey (on the root window I think). Whenever a plugin requests to bind to a single modifier key, Compiz has to also bind to modifier+everything_else so as to be able to distinguish between the modifier being tapped (for the plugin) or being used as a modifier for some other key. This only happens with the unityshell plugin which likes to use Super and Alt for the Dash and HUD.

So yes, it sounds like Unity is probably blocking other apps from binding to Super+anything on the root window. And I'm not sure if there is a safe way to avoid it without unbinding Unity from the Super key completely.

P.S. XKB is not a solution. It causes more problems than it solves because it does not honour active grabs. Hence bug 806255.