Is there a way to make a "multi-layer brush" or something like that?

[Ofnuts]

A technique to try is to make three brushes (all with the same rectangle size), one with the trunk, one with the leaves, and one with the shadow.

You then create a path(*), and you stroke the path with each brush on three different layers.

   

(*) the path doesn't need to be a single line. It can well be a set of very short strokes as long as these are shorted than the brush spacing. They can even be single point strokes (but this requires on of my scripts to render it).