Is Botox applied to the trapezius to reduce its size a procedure that can be (legally/safely) done in the US?

Never heard of that application. In theory, that would work but in practicality, it would be dangerous as the trapezius muscle is a large and very strong muscle such that the amounts of neuromodulator it would require to decrease the muscle bulk would be unsafe due to the risk of causing botulism. I would advise against that.

A large muscle such as the trapezius would require large doses of neurotoxin to effect A change. In my view this may not be safe and may lead to botulism. Hope this helps. Dr. Joseph.

Injection of the trapezius with Botox is not an FDA approved indication. it would probably work but would take a great deal of Botox to get any kind of result. discuss with a board certified neurologist