Several hours or day before serving, prepare Balsamic Viniagrette: In pint jar with tight-fitting lid, shake all ingredients. Cover and set aside until ready to serve.
About 1 hour before serving, heat indoor or outdoor grill to medium. Coat grill rack with nonstick cooking spray. On cutting board, finely chop 2 plum halves. In bowl, mix chopped plum, the onion, vinegar, honey, cumin, salt, celery seeds, and pepper. Lift skin on breasts of hens and spread 1/4 of plum mixture under each.
Arrange hens, breast-side down, on grill rack 6 inches above heat and grill until golden brown--about 20 minutes. Meanwhile, thread remaining 8 plum halves on several thin metal or soaked bamboo skewers.
Turn hens and grill on remaining side 15 to 20 minutes or until juices run clear when trhigh is pierced with fork. Brush plums with some of Balsamic Vinaigrette and grill 3 minutes on each side. Pour remaining Vinaigrette into cruet.
To serve, on large platter arrange spinach in an even layer. Place hens on top. Remove plums from skewers and arrange with hens. Pass Balsamic Vinaigrette.