If it is safe to use, yes. They take finger prints, and test for drugs, why not for affiliation, and, as many view lying as bona fide, yes truth serum is legitimate, as long as it can be used safely... if it causes problems, they would use that reason to avoid taking it.

If the polygraph is safe, and does not cause any side effects, yes, it could be a legitimate part of an interview for a tax funded position.