I have a Gravity Works contact printing frame that comes with UV
transmitting plexiglass. I have used it for making platinum prints and
found no significant difference in printing times between it and glass.
I was using unshielded black light tubes for a light source. There may
be enough UV blocking in the glass of the tube itself that it wouldn't
make any difference. I never tried it with sunlight where their may be
more of a printing speed difference.
One major problem with it is the plexiglass is not rigid enough to provide
good contact between the negative and paper. There were bubbles of
unsharpness in the prints where there wasn't good contact. I wouldn't
recommend it.
John Sparks