Just .. get. .. a reservoir already !!!!
Do it do it do it do it do it!!!!!!
But for the sprinkler valve, I was assuming that you had an RO/DI hookup the same as a garden hose or washing machine. If that were the case then you could just get a sprinkler valve with the garden hose hookups, and you just hook it inline between the RO/DI feedline and the tap. If that's not how you have it, then you'd have to, I guess, get a sprinkler valve with PVC hookups, and then get an assortment of various PVC bits and step down the piping from whatever it is (probably 3/4" or 1") down to the 1/4". You'll have to get some of the pieces from Home Depot/Rona (the valve), and some of the pieces from Western Pool and Spa (because I don't think HD/Rona typically carry bushings down to 1/4"
). Bit of a PITA this way.
Do the reservoir -- really, don't wreck a membrane if you don't have to. At $100 a pop I can think of better ways to spend money.
If you need a hand with it, let me know, maybe we can work on something together..