Hello, I'm not a orthodontist but I do know a little bit about developing the musculoskeletal system from personal experience (bodybuilding), and I would like to throw in my two cents on the subject, so I'd like to propose a training regiment as it pertains to the musculoskeletal system in the face.  I'm not sure if it will help at all, but it's an idea.

  From what I understand the tongue is composed 8 muscle in total, 4 intrinsic and 4 extrinsic. Your intrinsic tongue muscles are  in your oral cavity(your mouth), and thus are likely to be the determining factor of your maxilla width, and partially your maxilla position. Your extrinsic tongue muscles are connected to your intrinsic muscles, your temporal bone (part of your skull) and your hyoid bone (just above your Adam's apple), this group I assume to be the determining factor of your maxilla position. So with that said we should be able to use resistance training for these groups to increase the static strength of the tongue which in turn should help with total development of the face.   (In theory)

  Reps and sets are a hallmark of resistance training and I will be using this framework for my proposed training.

Rep defined -  Start with a suction hold, and your teeth lightly together, (your jaw is going to act as a counter force in this situation).  with the tip of your tongue apply force directly on the point just behind between your two front teeth, begin to slowly roll the force toward that back of your mouth widening the tongue as you go. Continue untell the force is in the back 1/4 of the mouth and tongue. Once here hold for a number of seconds then move the force back toward the front of your mouth. This whole series of movements will be considered a rep.

 Sets - a set is just a number of reps, considering the primary counter force is the skull and jaw I think 4 sets of 15 reps is probably more then enough. (Just a guess).

Rest, muscles need rest in-order to properly heal and grow, a general rule of thumb is 2-3 days in between exercises depending on the individual, so I think this can probably be applied here too. Obviously while resting it is important to maintain proper tongue posture.

On a side note doing this while in the mckenzie chin tuck may offer more restive force and thus be optimal for this exercise.

   I know this is similar to hard mewing, but i feel this would better target the individual muscles of the tongue thus help to prevent imbalances and it provides a templet of when to exercise and when not to.

 Of coarse I'm not an expert by any means, and any input (especially from someone that some formal training) would be greatly appreciated. 

   edit:   The first one might of been a bust, however I did discovery a involuntary action that the tongue does that uses nearly all of the extrinsic muscles of the tongue.   It feels as though it pushes the lower mandible forward among other things, just imagine every muscle in this picture being worked.

It's really easy to do, but it's uncomfortable.

  Just lay down on your back on a flat surface without a pillow. Then place your head in a mckenzie chin tuck position. And then do your best to relax all of your muscles. You will begin to feel discomfort in your jaw, your neck and under your ears,  ( I also felt pressure on my nasal bridge) this is your tongue re-positioning your head. Just let it happen and don't fight it. Though it can be intense the feeling will subside once your head is in a position that allows for a good airway.