And, as I understand it, the FAA has really tightened up on "known
icing certification since the Mooney 231 was certified.

True, but that doesn't effect existing planes. In fact, the weeping
wings system that Mooney uses is still in use by Mooney to produce
known-ice singles. The only change since original is a POH update
mandated by an AD that says you should attempt to get out of ice. The
nice thing about the weeping wings is that is protects a lot more of
the wing than the small area boots protect.

-Robert