To window class Progman, send a WM_SYSCOMMAND (0x112) message with wParam = SC_MONITORPOWER (0xF170) and lParam = 2. I'm using the HoeKey hotkey program with the following code:
^_`=Wait|1000
=Msg|Progman|274|61808|2 ; turn monitor off
There the numbers are in decimal, and there's a one second delay first so key releases don't turn the screen back on.
It's possible to add them for many native apps.
I have global media keys in Pandora and iTunes (standard shortcuts, not the dedicated keys on a keyboard) through use of the hotkey app Hoekey