We did that in the 60's in the ymca coursre. This is rumor,,,,,, but I heard that those skills stopped when bcd's came out. Back then your rig was some what of a back plate set up with out a wing. We wore a horse collar. You loosened the waist strap and took the rig off over our head while beathing and put in on the bottom with your weight belt over it to keep it in one place. You shut off air and went to the edge of the pool. to put it on you dove in put on your weight belt turned on air and breathed. then put the tank on over your head, rolled and secured the waist belt. We did ths with out a wet suit on for obvious reasons. Today with the mass number of hoses and crap with a jacket and very buoyant wet suits it is just too difficult to do that. with the BCD you are better off inflating to get to the surface than you are ditching your rig. After all ifou cant get out of the water wit your rig you float it not sink it.
Can horse collars with 2 bladders like aquatec be used for technical diving ; below 40 meters.