I’m assuming the valve connection is either threaded or a bolted flange. If you don’t mind getting soaked with some really stank water, simply remove the valve and quickly replace it with another valve. Leaving the replacement valve open and letting the water run thru it while installing it will mitigate the water pressure against the valve. In this scenario, a flanged valve will be much easier to install than will a threaded valve, but both are doable. At full pool, the water pressure is probably no more than 10psi............. That's probably what I'd do, but then again, I'm about half crazy.
If it is a threaded valve, there is a possibility that removing the valve may reveal damaged threads on the PVC pipe, (this often happens when someone uses too much Teflon tape on PVC pipe threads) therefore it would be wise to have a saw, pvc glue, a coupling, and a replacement threaded nipple at the ready.
They make freeze packs which utilize CO2. A freeze pack could be used to freeze the water in the pipe next to the valve, creating an ice dam, which would allow replacement of the valve without having to deal with running water. A plumber could provide this service, but that would probably cost more than hiring a US Navy dive team to place a cap on the inlet........... Just a thought, but I wonder if wrapping some small blocks of dry ice around the pipe would work........ I'd be willing to try it, that'd be cool!
