Usually mocha can handle face tracking without track marks and you can avoid having to remove them after the fact. Keep in mind that mocha will give you a "flat" surface, but like you say using warps or displacements will help create the illusion there is depth.
Depending on the motion, you might be able to use the new mocha AE v3 features to get a 3D camera solve and then use FreeForm to do true 3D mesh warp around the face.
You may also have to break the face into multiple surfaces (one for front and sides) depending on the motion.
If I were you, I would avoid using "real maggots" and use a particle system such as Particular with a good source animation OR 3D modeled and animated bugs.
It is doubtful that shooting on a mannequin would really help you here as the motion will not match the actors. If you have the budget & time, using a more sophisticated 3D tracking system like PFtrack's geometry tracker would be the best way to do this kind of effect, but I do believe that mocha and some ingenuity will get you there.
All the best,
Ross
Ross Shain
Imagineer Systems
http://www.imagineersystems.com
