Similar question came up the other day on LF Forum, and Bruce Barlow noted that Richard Ritter uses plexiglass as the standard 'ground glass' in his ULF cameras. Sand King jumped in to say that the experience he has had with a RR camera is that the plastic GG is just as good as regular glass, plus it's lighter in weight and unbreakable. The only downside is that plastic can be attacked by some solvents (which probably shouldn't be allowed within five miles of a good camera anyway), and can scratch.
Ritter gave a presentation at the 2005 LF Conference on quick and dirty field repairs in which he suggested that if you break your ground glass while traveling, just whip into a Walmart to buy a cheap plastic hoto frame of the appropriate size, and a package of fine sandpaper, then sand the plastic from the frame to make a replaceable ground glass.
I purchased a fresnel from Calumet for my Zone VI lightweight. The fresnel/ground 'glass' is actually plastic, but there is a thin cover glass to go over it. But the image is actually formed on the plastic fresnel/ground glass.