Better than letterbox we have a channel KENW from Roswell that broadcasts PBS in squeeze mode. This lets you stretch it back to 16:9 with a little loss of quality and certainly better than LB being zoomed on both axes, this just requires stretching X.
I found this when our PBS KNME stopped broadcasting OTA the Nightly Business News. KENW broadcast ARTS overnight. With no guide for KNME 5's companion KNMD 9, it is hard to guess where they may have put it--I should just look at their subscriber's monthly paper guide, but that takes some thought.
-Ken