Sounds like a good idea. Doesn't have to be invented from scratch; adapt a few psychological or behavioral-economics experiments. It's hard to ask about rationality in one's own life because of self-reporting problems; if we're going to do it, I think it's better to use questions of the form "Scenario: would you do a, b, c, or d?" rather than self-descriptive questions of the form "Are you more: a or b?"