You can certainly set it up in an ethical way. For example, tell the subject that they have to find something as fast as they can. It could be a set of drawers and a large bin nearby. One could deduce their (admittedly sunk-cost biased) intuitive probabilities from where they start looking and when/whether they switch from looking in the drawers to the bin. As described, this would not be easy or clean, but you can certainly modify the experiment to achieve both.