Sure that kind of NRV collet would work fine! Especially if it would be strong enough to cope opening of stubborn NRV. Getting 'NRV nut/head' fitted collet from a shop could be difficult (=impossible?). Very few people/companies have machines & know how to make special collets (at least to make collets from special strong tool steel & harden them to last).
i had a leaking nrv in a primus 96, I tried the hot water in the tank trick but the head of the valve was too mangled and the wrench kept slipping, I asked a colleague if he could drill out the valve with a pillar drill as desoldering the pump tube is not happening - no way - he suggested a different approach I wrapped many layers of tape around the wrench shaft to keep it centred in the pump tube and applied a blob of epoxy glue to the end of the wrench, pushed that over the damaged valve and left it overnight - next morning a good twist and the valve came out easy along with a lot of dirt from around the valve
