Are botox injections to masseter muscles (to ease TMJ pain) covered by medical insurance?

One can never predict what an insurance company will do. I have had many "discussions" with them over the years trying to get them to pay for thingsthey should and they should pay for this. You might try calling them first and be sure you are seeing a board certified dermatologist, plastic surgeon or dentist and not a medispa

Highly unlikely and it would be more efficient and relevant to actually get your insurance to answer that question than random doctors from around the country.