You can assign any action to any button! You only have to edit the skin.ini file. It cames with every skin, just rename the skin file from "name.bsz" to "name.zip", unpack it, and then insert in button's action position the code: 33!
Example:
Btn5=BPause,122,16,0,21;Pause -> Btn5=BPause,122,16,0,33;Cycle Aspect Ratio
Hope this could help!
Take a look at the Base skin skin.ini file and at the skins' forum! :)