Gabe Just so that I'm following. . .
The "fascia" issue you are encountering is when you are using the zCloth as a means to help you model the inner surface of the fat layer, correct?
Or are you actually running a separate zCloth simulation for the fascia - if so, I would recommend ditching that for the updated approach where we instead simulate the behaviour of the fascia by means of a zMaterial that we apply to the inside of the fat that uses both surface tension and pressure to both shrink wrap the fat to the muscles and then recover the peaks and valleys on the muscle model.
If the problem is happening just while using the zCloth as a modelling aid - then that is a different issue - as you'd need to animate the surface tension, or rest scale while applying pressure over 30 frames or so with collisions active and the open ends constrained to get the effect of the "air being sucked out" of the fascia so that it conforms to the muscle model.
I'd also recommend attempting to solve your muscles and fat together (either in pieces or globally) to achieve the best results (though at the cost of longer simulation times)
I hope this helps!