Mars atmsohperic dust loading over time should long since have been pulled out of the Viking ground shots, but as it is...

Well, the VL calibration targets turned out to be flawed, making it hard to get objectively reliable measurements of color on whicht to base that kind of compensation. However, since the Landers were on the ground through a winter there are several images of frost on the ground, and I once did an experiment based on the idea that the frost must be white, and therefore a meaningful calibration reference:

http://mars.magnificent.com/places/landings/viking/2/21I091/index.html

Apropos what you're saying, that image has been criticized a lot because it subtracts the effects of the atmosphere, thus it's not "realistic". I have to laugh at that sort of thing, because color perception is a subjective phenomenon of the human mind, and standard definitions of color names like "red", "green", and "blue", are really only a consensus derived from testing human subjects and asking them what colors they see. I'd expect eyes evolved on Mars to define "white" in terms appropriate to Mars, taking into account Mars's atmosphere. Martian eyes would see the frost as "white".

Isn't the fast and dirty way to handle MER images simply to set the average Matian sky to a known value (D65 white)?

No, I don't think so. The color of the sky seems to vary over time and with azimuth angle, and changes with dust loading. Without dust it ought to be some shade of blue due to Rayleigh scattering, but we don't see that often (and images portraying a bluish sky just set off more bitter controversy). In general I'm cautious about any "calibration" that's based on an assumption of the color of some object in the scene, unless it's something like a calibration target that's been thoroughly profiled in a laboratory. (He said despite having talkied two paragraphs ago about calibrating to frost--I don't claim to be consistent! It's just that some bets are safer than others--fresh frost is very likely to be white, but who can say what's going on in the atmosphere in any particular image?)

Hmm...ya know, I hear they're going to try to get the two MER rovers through the winter. I wonder if they'll wake them periodically, and maybe we'll see frost in some images? That would be very interesting with the better instruments on these rovers--they could determine the chemical composition of frost.