Grrrrr I hate it when I accidentally close a tab I'm typing in and lose everything I posted so far...
Anyway... someone mentioned in the other thread, removable mylar film for the front glass... that's actually an idea that's been used for decades in motocross, with tearaway strips adhered to helmet visors especially for riding in muddy conditions.
And I recall seeing an product some 25+ years ago in a motorcycle magazine, for a motorized system that moved a strip of film from a roll across the visor... found one current product like this, called Mud Muncher.
And looking at that link, they actually have a prototype custom housing that extends this concept for sport POV cameras:
The Camera Mud Muncher is a self contained motorized film guiding system made of billet aluminum. 2 mil thick film is moved across the lens using a 12 volt DC gear motor. With 15 programmable durations you can choose the right timing sequence for your application(optional). The Mud Muncher is used to achieve optical clarity in adverse conditions such as, but not limited to: rain, snow, mud, dirt, bugs or any type of airborne debris. Optical clarity can be achieved using the Mud Muncher in front of many types of optical processes, but is not limited to: your eyes, video cameras, cameras, visual surveillance systems, optical guidance.
I would expect, for a reasonable cost, they'd be willing to custom-build something for most CCTV camera enclosures. The period between "clearings" is programmable, so you could probably set it to only move once a week, or whatever the conditions dictate.