It depends on whether you want to simply draw something on top of the skeleton at a specific bome/slot location, or if what you draw should actually be inserted into the drawing order of the rendered attachments making up the skeleton.
For the first case, you can simply call skeletonMesh.skeleton.findBone("bonename").getWorldX()/getWorldY() after you called skeletonMesh.update(). This will give you 2D coordinates of the bone relative to the skeleton mesh position.
For the second case, things are quite a bit more complicated and there's nothing out of the box in the spine-threejs API for that. I'd have to put some effort in to provide something easy to use.
