The Commercial Diver Network
They use to run scuba tanks upside down on at least one model of the Scott Hydro Pac it was a full face rig in the mid 1950's to the 1960's. You could reach around and control the valve on that rig. Pretty cool rig the full face mask had a regulator on the side of it. You can most likely see one on line on a historical type page. Scott Hydro pac.
Just how many hats don't have a EGS? I can think of the Gorski, Swindel, and Desco Pot.