I think you should be able to make it work. I do something sorta similar. I've never actually used my RO for drinking, but I do T off the RO/DI before the DI stage to go into another reservoir which I use for watering my orchids. Some of the fussier orchids need to be watered with RO water but you don't want to use RO/DI for this, just RO. Anyhow so I "T" off the line going from the RO membrane to the DI canister into a rubbermaid type thing with a float valve. So when that thing is not full, the "priority" is to refill it (although some water makes it past the DI so the RO/DI output isn't completely "off" but it is "slowed"). When it's full, it stops filling and all the water goes to the DI.
Basically with the autoshutoff kit, you can have as many outputs as you want, as long as they go to some kind of shutoff (float valve, typically), when they are all closed, the output line will pressurize and close the input valve. If any one of them drops, the output line will depressurize, and the input valve opens and away you go.
Basically it's full automation without any electrical components, it's actually kind of neat.
