The EAA asked a while back about such things and the FAA put a lot of conditions (pilot minimum hours, etc...) on such subsidy. I don't know where it officially stands today. Philips66 does have a fuel rebate program. Frankly, anything that doesn't go to the pilot, is outside the FAA purview (so if they want to pay for publicity, refreshments for the volunteers, etc... ) can't see how that is a problem.