Glass etching suggests mild acidic content, which can occur when some bacteria permanently colonize cooling towers.
I don't have a professional solution, but kitchen cling film is transparent and acid resistent. If your camera was in an outdoor container with a flat plate glass aperture, it's possible that you could stretch the kitchen cling film across the aperture with negligible optical effects. Not sure how you would attach it to ensure it doesn't fall off... a criticism might be additional dust collection, but although cling film clings well to itself, it's not particularly susceptible to dust.
I think I've seen (on this site, somewhere) discussions of a similar transparent plastic treatment for cameras, that allow removal one layer at a time when the face becomes dirty. Although my guess is that you don't want to be removing anything, its plastic is also likely to be acid resistent.
IPVM has a few discussions that, while not directly relevant to acid etching, offer thoughts on the subject of protecting lenses:
Hope this is helpful.
Is there an outdoor housing sufficently air tight to prevent the vapor from entering the enclosure and etching the lens? This sounds more like an explosion proof enclosure with a window either specially treated or requiring a window material that is optically clear in the proper wavelengths and acid resistant. This sounds expensive.
We haven't been able to find a suitable enclosure, in terms of glass that will withstand the corrosive effects. Hence, the continuous air flow proposal. By following one of the links provided by Horace Lasell, I got as far as the following post:
"Undisclosed (#0323731) Manufacturer:
"Dust" and "industrial" bring up yet another idea: Thera are a few enclosures for machine-vision cameras which use a wind curtain to keep dust and other stuff from even reaching the glass. Problem is that you would need a source of pressurized air."
I have sent a request to "Undisclosed (#0323731) Manufacturer" for more information, but nothing back as yet.