Media keys (such as volume up/down and eject) are not handled by the keyboard subsystem of Mac OS X (they're handled by BezelServices via the generalized HID subsystem). In result, both their recording and playback as keystrokes isn't supported by OS X (=> these keys aren't submitted over OS X Screen Sharing sessions for the same reason).

Since Apple doesn't provide any public API for putting monitors to sleep or offers any way to configure the keyboard shortcut for this functionality, either, I'm afraid I can't help you in this case. Sorry.

Best regards, 
Felix Schwarz