I'v not seen a subluxed SC joint, I have however had stiff SC joint myself when I was 16 - 21 while I was doing alot of competitive rock climbing.

I used to manipulate my SC joint using ext rot at 90 degree abd and a deep breath in - I was taught this by a physio manip tutor as he suffered the same problem after heavy weight lifting and body building.

My thoughts: As we know the SC joint moves with the GH and AC joint - If the scapular is correctly stabilised and humeral movements are co-ordinated correctly then the SC joint will be in the best position to heal - so I feel the postural work that you are doing is well indicated. And perhaps you can look at - following how my and my tutors SC's had became stiff - a high level of strengthening around the shoulder while maintaining scapular position. I'm not saying it would work - as it's already subluxed, unless the strengthening can be done without causing excessive mobility, it may just cause further mobility.

good luck - very interesting case and it'd be good to hear how you achieve good results.