Great concept... Maybe mtpaint is a good candidate for the task since it is fast and already supports alpha and pressure sensitivity? Gimp and Krita are out of the question - Gimp is too big and Krita is KDE/not mature. Mypaint seemed slow when i last tried it, maybe it is getting better?

Link to mtpaint:
http://www.internettablettalk.com/fo...ad.php?t=17712