directing the propeller wash from his scooter downwards towards the bottom. It is a relatively common technique for removing layers of sand to look for artifacts in the vicinity of wrecks. It no doubt has other uses as well, but creates a bit of a 'blizzard' of sand around the diver.
JB -
One thing you might try, don't have any idea how useful it would be. Some companies manufacture small nylon bags that thread onto the hose of a regulator, and velcro in place over the second stage. They are ostensibly used to isolate the second stage of a regulator attached to an O2 bottle, so that it is not mistaken for another stage or deco regulator. Given the somewhat unique circumstances of your diving, such an arrangement might keep some of the particulate out of your second stage. It would be worth a try, so long as you are able to get it off in about a millisecond should you ever need your backup regulator.
Doc