> Trying to keep a random number secret likely is less stressful than an actual lie.

Which begs the issue: If a lie detector is accurate only for answers that are stressful, then it sure looks like we need a detector to figure out if an answer is stressful or not.

Or to put it differently, now you have two problems: Lie detection and stressful-answer detection.

Nevertheless, I concede that your beer detection plan is superior.