I've never heard of that - I don't think that's truly "industry standard"!
I had a neurotologist and a neurosurgeon, and both were paid for by insurance. Is it possible the the assistant is considered out of network? I know out of network providers are usually not paid for in full, but still, they should pay more than 20% if that was the case.
If you're having translab, I'm pretty sure you have to have a neurosurgeon - the ENT or neurotologist does anything outside the dura mater (covering of the brain), and the neurosurgeon has to do anything inside.
I would talk to your doctor's office about it. You have to remember that the majority of insurance company employees have no medical training, so they have no idea what's standard or not, especially for an uncommon surgery like this. I've experienced this a lot myself. I would definitely appeal their decision. 80% of brain surgery could be very expensive!
Good luck.
Lori