We can apply the principles of Yin Yoga to any joint in the body, including the shoulders. Recall, the main principles of Yin Yoga are to - apply a stress, don't move, and let it soak in for time. Our connective tissues, being yin-like relative to our muscles, in the context of mobility and elasticity, are more safely exercised in a yin-way than in the more active yang-ways. There has been a couple of discussions in the forum in the past on Yin Yoga specifically for the shoulders, and to read them you can click here:
However, you are talking about a particular situation with your shoulder where a pathology exists. Is Yin Yoga a good idea for someone with Shoulder Impingement or Capsulitis? First, for readers who are not familar with these problems, a nice semi-technical description can be found here.
Why not read this article first and then come back to this discussion. As the article describes, there are many possible causes for impingement: "Causes of impingement include acromioclavicular joint arthritis, calcified coracoacromial ligament, structural abnormalities of the acromion and weakness of the rotator cuff muscles. " Treatment depends upon the severity of the condition, but generally includes mobilization of the joint with moderate muscular activation (for example, the exercises recommended are low weight/high repetition movements in one of three directions - internal rotation, external rotation and abduction).
The arm is not immobilized or put in a sling, because immobility can lead to capsilitis (frozen shoulder syndrome). Based on anything I have read, I do not believe that Yin Yoga during the rehab period would be wise. Gentle yang-movements are indicated. After the joint has healed, then perhaps some yin-type exercises, as described in the above noted posts, may help to strenghten and thicken the ligaments, helping to avoid future problems. But while you are healing, I probably would suggest sticking to the program your physio is recommending.
Hope this helps.