I'm wondering if it's possible to have a colored brush tip composited with a paint color. For example, I'd like to create a bristle brush that has some hue/saturation variation in the bristles, but which I can still easily change the base color for. Any ideas?

Yup! Check out Hue and Saturation under Source in the brust settings for the pixel engine!
http://imgur.com/a/ZFJUi

That affects the whole stamp, I'd like to do something like this:


This example would be for a dual brush setup which I know isn't possible at the moment. But I'm wondering if it would be possible to take a colored stamp and instead of the stamp's color directly driving the painted output color, composite that color with an user-selected color and then apply the alpha of the colored stamp.