Pumping with cock ring
I have tried it and it does work. I use a metal CR which is mildly constricting when erect. I tried it out with a flanged Le Luv cylinder and my mr fantastic air pump. The only issue I had was that it was hard to remove while I was in a pumped up state. Also did not feel it was a good idea to leave it on for an extended period.
The mr fantastic water pump comes with special unflanged cylinders that fit into a silicone sealing ring. That ring works just like a CR compressing the shaft so I see no need to also use the metal CR. I sometimes leave the sealing ring on for a little while after pumping to prolong that chubby feel.