What metal OK for RO water?

[midgetwaiter]

You are making this harder than it needs to be.

Your membrane flush is on the waste water side so you never come in contact with RO and it goes down the drain anyway, so it doesn't matter.

In order to switch the RO on and off to fill your holding tank I'd seriously suggest switching the input water. If you switch on the output side you will need to use an RO safe valve and you have increased leak potential because the unit will be pressurized. There's at least twelve points on a pressurized RO/DI unit that could fail but it you shut off the feed you only have one fitting that has any pressure.