We've done that as well. My mom likes to get the chips the foodcourt serves with sandwiches as a snack and take them for before the flight. They give them to us on a plate with a clear plastic lid.
We had quick service and snack credits left over on our last trip, so we got sandwiches and chips, fruit cups, etc. at Earl of Sandwich and ate our last Disney meal at the airport.
We totally did this and got fun things for the ride home after finding we had like 8 snack credits the evening before we were supposed to leave.
I tried to do the whole "snack credits for breakfast" thing by getting a cinnamon bun from Main Street until the guy serving scooped extra sugary topping on my steaming hot cinnamon roll and I had a moment of weakness... Yum.
We are not snackers, so always have almost all of our snack credits left to use up the last day. We usually use them at Goofy's in DTD, the Confectionary at MK, the Werther's Caramel Store in Germany, or Seashore Sweets at BW.