Hello,
Are there any alternative postures/gestures that will give the benefits of yoga mudra? I can join my fingers briefly, but kneeling.
I do not understand the second segment of the post. You can join your fingers briefly but kneeling?
Hi,
What I meant was that if I am kneeling and upright, I can join my fingers with a bit of effort. If I am sitting on the floor,however, the plot thickens. it becomes much harder and I can’t join any fingers at all. It’s as if I’m reaching back around a thick tree. I do have a few extra pounds, but it seems more a flexibility issue. 
Have you tried using a strap?
So you are referencing this posture?
Why have you added or selected this pose? For what effects please?
Why are you bearing weight on the head in this way?
Have you been instructed in how to do this pose?
These may remain as rhetorical questions (if you like).
Anatomically you ask about mobility of the shoulder blades (not exclusively but in an applied anatomy context). Of course said mobility will change from kneeling to the full pose with the head on the ground. If your question is simply "how do I do this pose with my current mobility restriction" then you have an answer above - use a strap or towel grasped by the hands.
If the question is different, let me know and thus the answer will be different also.