It could easily be done with a vending machine. When you pick up your permit you leave $100 (or whatever) deposit on your credit card and get your WAG bag. When you finish your hike you go to the machine at the trail head (or close by) and have your WAG bag barcode read. It then deposits it in a tamper proof bin designed to be easily swapped out and hauled away. Your card is credited back the deposit. You leave your bag somewhere, you lose the deposit.
Excellent idea, DUG!
