Awesome suggestions, thanks so much! I wish I had seen those Rose Brand hinges earlier - those would have been perfect.
We wound up going with piano hinges mounted on the edge of the door and the trap opening. Worked pretty well; with the luaun "planks" added to the tops hid the hinges well enough to not be a trip hazard or interfere with the door opening. Had to scrap the original design for the doors themselves because plywood doesn't like a zillion little screws in it on edge. Imagine that. Went with solid 2x8's pegged together. Heavier, but rigid enough to not need additional framing.