One tip for skin weight painting secondary joints is to first skin a simplified version of the mesh then use Maya's copy weights tool to transfer those weights the more complex mesh.
For example i needed to skin some lip secondary joints so I:
1. duplicated the skinned mesh
2. selected edgeloops around lip of the duplicate, converted selection to faces, then split faces.
3. then i drew secondary joints for this simplified mesh and skinned this mesh to the secondary joints. It was helpful using max influence of 1, and closest distance settings.
4. last i transfered these weights to the original skinned mesh by first selecting the source weight vertices then the destination mesh vertices then i used the Maya copy weights (one to one).
https://gist.github.com/nathanielanozie naSelectVertsByWorldPosition.mel
Hope you find this helpful.
Happy 3d sketching,
Nate
