How do you assign multiple shrinkwrap modifiers to rig muscles? I'm in the middle of porting my rigging tools from other animation software over to Blender and was wondering how to use the shrinkwrap modifier to achieve muscle deforms, such as what JP Bouza did in BlenRig 3.  Right now, I really don't know how to apply multiple shrinkwrap modifiers to a model to achieve this.  The modifier stack doesn't work in parallel (yet; this might change with the new Depsgraph, but not right now).  So, the effects of each shrinkwrap modifier cascades over to the next in the stack.

Is it s magic combination of vertex groups?  If so, how does one weight paint to each shrinkwrap modifier?
What I do for muscles rig is that I have 2 Armatures and 2 Meshes:


All muscles as 1 mesh with rig
Character mesh with rig


That means no muscle groups and no clutter in scene and character and only 1 shring-wrap modifier.


The muscle rig bones are parented to character rig with Child Of constraints. This also allows for muscle layering.
The individual muscle jiggle is controlled with 1 vertex group for soft-body goal.
Each muscle has its own shape-key controlled with muscle-rig bones.
Character mesh has only 1 shrink-wrap with vertex-group only for muscle-bulging strength.