The answer is that it depends on your insurance, and the reason for it's use (indication). Some insurance companies consider botox with this diagnosis as cosmetic or non-functional, and deny all coverage. Other companies may go the opposite direction and just assume coverage.
But most will look at the specific indication and/or require more information to determine that specific indication. That's because they want to ensure the reason for botox use is functional. Issues resulting from Bell's palsy such as eye protection, drooling, biting the lip are all functional problems. Therefore if your physician is clear in treatment for these issues, coverage is much more likely.
My suggestion to you is to start with a phone call to your insurance company benefits or customer service line. They should be able to tell you about coverage, level of coverage, or even a request to preauthorize your injections submitted by your surgeon. And please see a surgeon (plastic surgeon, facial plastic surgeon, or oculoplastic surgeon) for this type of care.