Actually, I believe the answer is no.
The anti-kickback statute applies if the service or item is "reimbursable" by a federal program (e.g. VA, DOD, CMS, Tricare). It doesn't specify that the service/item is actually partially paid by medicare at that time. Since the point of a "free-trial" is to get the patient to continue with a drug that would be subsequently be paid for by Medicare the statute would apply.
Exceptions are made when the company pays for all of the cost of the drug throughout treatment (or only within the doughnut hole). These are usually Patient Assistance Programs. I don't believe the one time use cards ever count.
Most of the free drug coupons state if Medicare patient's are eligible. I would advise you always follow the instructions on the card, or call the company.