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.

At least from my own experience with MH fixtures, there's a good
chance the glass is tempered, designed to protect you from excess UV
and from showers of nearly-molten quartz if a bulb ever detonates. I
would not mess with the factory glass.

How hot does the outside of the glass get?

I used to have an MH in my garage that was a high-bay fixture in a
lower-height room, so I put an external diffuser on it, just cut a
circle out of one of those plastic diffuser sheets you can get at the
hardware store for 4-tube fluorescent fixtures, and mounted the
circle an inch below the MH fixture's glass.

--
is Joshua Putnam
http://www.phred.org/~josh/
Updated Infrared Photography Books List:
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