Scott - not sure if any of the following is relevant, but it may be. My son needed a cochlear implant and my self-insured employer was not supportive at first. Here's what ended up to be successful...
First, we sent a certified letter to our benefits office, indicating that we'd engaged an attorney. Second, we provided written justification regarding his (my son's) specific procedure, including long-term benefits and associated risks. For a THA, this should be pretty easy to do. You should be able to find numerous case studies, so that the procedure will be easily confirmed as medically necessary and not experimental.
Our office told me that they were sending my documents to an external review agency. I assume that you could request that external review without them offering it. At no time did they indicate that further legal action would be restricted by this procedure. The external review agency reviewed and returned a "positive" decision".
We then went through the process to get the implant manufacturer recognized as in-network, since there aren't any of these providers locally. That gave us basically in-network deductibles and the like. Surgeons and specialists were already in-network...
Once these steps were finalized and documented, we were able to get his surgery scheduled.
One of the most critical steps was finding a contact in our benefits office, who handled everything. This person is now a valuable contact who's asked me for information about other people here who are looking at implants. She's a "special case coordinator" or something like that.
Our insurance agency is Highmark Blue Shield, if that makes any difference. Ours was a "procedure" question, yours appears to be a "not qualified provider" issue. I'm not sure how that plays in this situation.
Best,
Jim