Hi ya alxpix
Personally I treat clients with Indian head massage and other techniques. Even though you have to becareful with treating frozen shoulders, working them regularly is really important.Shoulder problems need regular short treatments, as there is a tendancy for atrophy and further reduced mobility if the problem isn't sorted out. Some IHM practitioners consider frozen shoulders as a contraindication, but that's rubbish.
Kneading and frictions to the shoulder and upper arm can be performed first, and then gentle mobilisation afterwards. You could also massage the occiptal where there is any tenderness (usually around the acu-point GB20). Also massage the fingers of the hand (same side as shoulder) and give them a good wiggle! This helps to move the chi/energy through the meridians associated with the shoulder.
Hope this helps.
Best Wishes
Dave the Reiki Pixie