I spent three days on "the playa" a few weeks back with a different kind of crowd... I brought my 4x5 Newton Nu-Vue and kept it in a simple "Jansport" backpack. In the lunch-type cooler I had Ziploc bags with the Grafmatic, 2 lenses for the 4x5, Spotmatic F spare body (never came out of the Ziploc), 35mm film, 135mm lens (never used it). Around my neck the whole time, ES-II with 50mm and yellow filter. I only used the 4x5 for 6 shots. The rest was 35mm.

Sure things were dusty, but it had rained the week before and there were only a fraction of the people you will have, so not a huge amount of kicked up dust airborne...

A vintage folder like your Mamiya 6 is probably a good bet. You know what to expect, sure dust will get into it but you probably can minimize it by keeping it behind a shirt or in a protective bag when not actively shooting.