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?