Anatomically the superior genohumeral ligament unwinds/become more lax in internal rotation of the arm. It would follow then that a flaccid arm, left in internal rotation would tend towards subluxation.

I have used various cuffs over the years and have even experimented with a few thermoplastic creations of my own. I must say that none have been fantastic and none are useful in isolation.

I have also tried muscle stimulation for supination of the forarm combined with external rotation of the shoulder girdle to facilitate a better boney alignment. OT and PT exercises that place objects and tasks to the side and behind the effected side would also encourage this.

Anyway that's a few ideas from me, best of luck 8o