Inspired by xenzag's idea. You could control these sounds or have them randomly change to inspire your brush stroke style. Done like this:
1- Contact mic takes any noise made from your paintbrush and...
2- amplifies it putting it through the following audio effects:
a) Echo. This could be appropriate
for etherial, spooky or dreamy brush strokes.
b) Distortion. This would be appropriate for angry or fast brush strokes.
c) Phase shift. This would be appropriate for brush strokes that need to sound like they're going through a phase shifter for some reason.
d) Noise gate triggered by various inputs, like people walking by saying "What's all that weird sound coming from your painting?" You'd only hear the brush noises when they were talking.
e) Detuning. Brush sound dropped two octaves to basically sound like an earth quake, or raised two octaves to sound like what a flea painting might sound like.
and of course, f) Any combination of those sounds.
Speakers would be behind the canvas so it it sounds like that's where their coming from. Feedback would be controlled by a limiting system for any audio engineer nerds out there.
Would also be interesting to just have the brush be the contact mic and actually expand on the feedback system having the canvas vibrate, its movement painting itself under the relatively static brush.