I have been in touch with a timber rattlesnake rehabber in NY and she was wondering if the lump in the black rat snake's neck that I am attempting to rehab is a bott fly maggot. But it does not look that way.

Did they say what it could be (any guess before results form the University)?

Bott flies lay their eggs inside the skin. It hatches into a maggot that eats some of the surrounding flesh and skin. The skin around dies away, the maggot comes out and goes from pupa to fly. The animal recovers, losing some skin, etc. around the area.

I am glad you caught it early, whatever it is. And good luck!!