You can look into Powerlaunch:

http://powerlaunch.garage.maemo.org/

It allows you to globally remap the power button to enter a special mode where the other hardware buttons have special meaning. So power + fullscreen could be pause; power + plus could be volume up, etc.

It also has a "pocket mode" for controlling music via some of the hardware keys while the screen and the rest of the keyboard is locked.

Be warned - its configuration can be pretty complicated.