Planks and all their variations (e.g. shoulder taps) are among my favorite ways for working on shoulder stability. In closed chain, you can really build up tension throughout the shoulder girdle. I usually like to use the verbal cue of "screw your hands into the ground" to facilitate scapular protraction and shoulder external rotation. Furthermore, being in closed chain allows the athlete to have greater sensory feedback and adjust to perturbations.
The reformer is a fantastic tool for closed chain shoulder stability because the springs facilitate an extension moment at the shoulder joint. You will inherently resist this moment through an isometric shoulder flexion force, with the goal being to engage the serratus anterior and "push up and out" against the reformer.
✅ "Screw your hands into the ground and push up and out" against the reformer.
✅ Push the reformer forward by using your SCAPULA. Protract and push them forward.
✅ Move slowly on the shoulder taps. SLOWLY unweight your arm. The moment you feel a loss of stability, stop and fix it - if you can't, regress the exercise to weight shifts.