This has been something I've wanted to figure out for a long time and finally did.

This uses a Surface Deformation on a base model with a cage model that has the actual shape keys on it, applying only to the painted areas.
Other modifiers move the model too far along the armature movements and look horrible or would get caught on intersecting geometry.

The shape key I used was just a basic expansion one, moving all vertices out a bit and limiting the brush to the legs, but it would work with everything else once I do some more work on the model.

Model is a mash of different renas I've found and am slowly cleaning up for other plans. Still needs a ton of work, but wanted to show this cool trick off.