Keybinding of modifier keys on a Mac

From what I gather, it is possible to bind specific modifier keys, such as "lshift" or "rshift" on Windows, and have it work. But when you try to do this on a Mac, nothing happens when the modifier key is pressed. (The idea is to have pressing *only* the modifier key result in an event.) If indeed it is supported on Windows and not on a Mac, then this is a discrepance between the platforms that can hopefully be fixed soon.

I'm using 1.5 Beta 3 on the Mac. Probably happens on all prior versions too, but I'm not positive.