I think this is important to stress. You, as a jumpseater - cabin, cockpit, or cabin jumpseat are bound by the rules. Many people consider you a non-rev if you are seated in a pax seat which is totally wrong. Having said that, I don't think the FAA is too concerned with dress, and any chief pilot or otherwise has too much time on their hands if you are doing the carpet dance for letting "Daisy Duke" ride in a pax seat. Rules are meant to be followed, I will concede that, however some rules are much more important than others. I would agree with the folks who give them the talk, ask if they have something else to wear, and probably let them ride with the knowledge that they will never do it again.
On a side note, I think the $12.50 is a pretty scumbag way to make money off of employees.