You know, around here (Ohio), Merial is letting regular horse vets without any special credentials buy and administer Oncept. All they have to do is buy the intradermal injector. This is a change from their earlier policies which did require a diplomate in oncology or internal medicine. It is nutty to me that they have apparently different policies in different parts of the country? I think I would have your vet run this up the chain of command (so to speak) at Merial and see if he/she can get a read that is more consistent with other parts of the country. Second choice is to try a vet school and see if they can help.
Good luck and let us know how it goes.