I tried doing something similar. Saw some soldiers at a buffet and I went up to the cashier and gave her $40, telling her that the GIs' money was no good there. She gave it back and said, "you're the third guy to try that."
Pretty cool. First from the standpoint that there were others ready to step up and second, she could have pocketed the extras and nobody would have been the wiser.
we usually corner our servers as soon as the serviceman sits down and tell them the meal is on us. We shake their hands and thank them as we leave, but don’t mention the meal.