If it's payment up front of a set fee for a specified course of treatment (for, say, cleaning twice a year and bitewing x-rays once a year or for something like a full course of orthodontia) and does not provide for discounted rates for services not included in the specified course of treatment, I would consider it to be the cost of that specific course of medical treatment and to be a qualified medical expense. If it's payment for a plan that permits you to pay a discounted rate for services that are not explicitly part of specified course of treatment (with some services perhaps discounted to $0), I would consider it not to be a qualified medical expense since it's a payment to receive discounted rates, not payment for medical service.
