Unfortunately for Massie, not only does the Constitution allow for Democrats to punish Republicans who don't conform (semi-sarc), but existing laws allow fines for members of Congress, although no mention of masks.
"Article I, Section 5, Clause 2: Each House may determine the Rules of its Proceedings, punish its Members for disorderly Behavior [emphasis added], and, with the Concurrence of two thirds, expel a Member."
Laws seem to be concerned with abuse of office.
So depending on judge and Pelosi's questionable knowledge of the Constitution, Massie may be out a few bucks.
Corrections, insights welcome.
The House has fined Members as a form of punishment, but this is an actual salary cut, as I understand it. I don’t see how a prohibition on increasing a salary during a term would not also logically prohibit any change of salary, up or down, but it’s not explicit so I guess they could get away with it.
I still hold that other sections that prohibit reductions in salary for the executive and for judges wouldn’t also apply to members of Congress, but, again, it’s not explicit.
See here for a history of Section 5 punishments: https://history.house.gov/Institution/Origins-Development/Discipline/