The problem with respirators is they require a seal around the face. Anyone who has ever done much out of position welding in confined spaces has had an errant spark come bouncing around inside their mask. I've even had them get inside of leather sock hoods not fun when you have something making a seal around your face the spark will always find a fold to adhere to creating a painful burn. without a respirator that same spark may have struck your face but rolled harmlessly away while maybe creating a small irritating burn spot. Also those of us who sport a beard would tell the constabulary to go sod themselves.
I've been inside pressure vessels and tanks where it gets really hot and often times they are dusty respirators even forced air ones do not do much to alleviate the discomfort except for being able to breathe. What I usually did was merely attach a long hose to the exhaust of a clean shop-vac to the top inside of my hood not only could I breathe but it helped to reduce the buildup of heat in my hood.
We were replacing the klinker chains in a cement kiln once the kiln was still 140°f inside piping forced cooling only served to create a thick dust cloud so we used air hoses from a large air compressor with a charcoal scrubber filter the air was close to dive tank quality which turned out to be a better solution than having us try to wear underwater type welding helmets too cumbersome and heavy when you are not in water.
So your idea or a personal waist pack with a turbine fan ducted through a hose to your helmet is a good one you will want to add a rear cover to fully engulf your head to insure the air flow has a positive pressure forcing the air down past your neck and shoulders.