If you want a reflective white diffusing coating, the standard optical
product is called "Munsell White Reflectance Coating". It's real good
stuff and easily coated onto glass. I suspect you're talking about a
transmissive diffuser, though (the other side of a piece of glass
coated with this stuff will have only a tiny fraction of a percent of
light shining through!)

And a colleague who used to work in the lighting industry informed me
that most frosted glass used to be done with hydrofluoric acid etching,
although he was of the opinion that this is seen as environmentally
unfriendly today.

Tim.