re: Why don't NASA's $800 million Mars Rovers have solar panel cleaners?
Maybe they can do something like that by manuvering the lander over a small rock or two instead of manuvering the lander around even the smallest rocks.
The Delta-V, "bouncing around" and small amounts of wind resistance generated might cause much of the dust to roll-off the surface. making a cleaning system relatively unneeded.
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The lander/rovers are so big that I don't think that they will be able to 'avoid' those smaller rocks. The shedding of Martian sand on the panels may 'naturally' come off as the rovers make their way, just as you said.
I wonder if there's a clip showing FIDO as he traveled through the Mojave desert. I know, FIDO was a smaller version, but there were no roads where he was either.