The artist Animtim I met at Amsterdam KritaSprint 2011 use a Joypad+tablet to work. He gaves me precious infos about how simple it was to attribute keyboard shortcut using the FLOSS Qjoypad. This ideas mixed in my mind and made me want to stick a flat gamepad on the Cintiq to replace default button. That why I decided to 'flatten' an old Logitech Precision Gamepad I had to try it. But, if you are not tempted by DIY stuff, a lot of mini gamepad flat exist on the market for very cheap ( exemple , Dicota mini gamepad ) , and if you want to try , you will surely prefer to purchase one and stick it on your tablet with adhesive removable gum.
I ordered one today for less than 3Euro, I will try it and adopt it if it's better than my frankenstein flat gamepad creation. Also , on the same theme I read yesterday this great review and this other one from the anime artist Saejin.
Here a video to showcase the first function I did for Mypaint with my flat gamepad 'creation'
( sorry for the video quality, this was shoot with a cheap photo camera with a 640x480 video function , not a real movie camera )