[Most (or some) RO units come with an ASOV (auto shut off valve). The saddle piercing valve merely punctures the rigid copper pipe, providing water to the unit. Depending on the maker, the unit or the valve after the unit turns off the water.]
Have you tried getting water from a water spout without a canister? If you don't have a reservoir of some type, it will be PAINfully slow. I can hardly imagine waiting for water without the canister/tank even though I currently run 2 100GPD membranes and a permeate pump. I thought about it for awhile too, but I'm glad I went ahead and got the tank (and made room for it). Just my $0.02.]
[It goes after the RO filter. It shuts off the waste water when it detects no flow on the clean water side. It's not a problem for the unit to stay pressurized when the ASOV shuts down. The units are built for it.]
[It will be very slow. I was all out of RO water (I keep bottles of it in the fridge for when I'm thirsty), so I opened the valve over a glass. It probably took a minute to fill up a glass.
I don't think you'll be too happy with this plan, but let us know later what your impression is.]
