Hmmm, Ok I was kind of worried you were going to say that. Having been down this road myself .. let me spare you some things I learned the hard way.
First of all, in answer to your question, what you want is a solenoid shutoff kit for your RO/DI unit. The float valve closes, the output line pressures up, and this pressure drives closed another valve on the input side of the RO/DI. Thus, when the output valve is off, the unit is off, no longer dumping waste water.
But !!
That's still not quite enough. The problem with this scenario is that the water will get replaced at the rate of evaporation. Which could mean that you end up having the unit on for like 10 seconds every 5 minutes. This is very hard on the RO membrane, I ended up going through 3 membranes in a year until I figured out what I was doing was killing my membranes (the RO would output TDS at 80-100 .. the membranes were shot).
So.. you need a sort of buffer in between the sump and the RO/DI. What I do is I have a bucket I use as a RO/DI reservoir. In my case it's like a 50g food grade barrell. I fill it up manually and let it fill overnight (it still has the shutoff kit so no worries about overflows), and then turn off the RO/DI manually. This lasts me about 2 weeks between fills. (It's still not a perfect arrangement - more on that later). Then, basically the float valve on the sump is attached to the reservoir. So the water can enter into the sump at whatever rate the evaporation is, and I don't have to worry about destroying my membrane.
Now .. (here's the "later" part I promised..) It turns out that it's also not great to have your RO/DI sit idle for 2 weeks at a time because you still get TDS creep. Right now I find myself having to flush the membrane every time I run the unit, which shouldn't be necessary.
What would be perfectly ideal is have a sprinkler timer on the input and have the RO/DI turn on for an hour each night. Thus it still has the benefit of a single run (ie., when it's on it's on, and the shutoff kit is then at that point a failsafe in case the bucket fills to the top), AND it has the benefit of not allowing the TDS to accumulate.
There is a clever design of a reservoir using a check valve that basically allows you to have it self-fill if (and only if) the reservoir empties completely. I don't have the URL handy but it was talked about on Canreef so there are some threads on it somewhere. I figure the hose/sprinkler timer thing would work fine too (I just have been too lazy to look for one at HD. I probably will when I get my basement further along.)
This is the shutoff valve kit:
http://www.jlaquatics.com/phpstore/s..._ID=ro-kpfshut
This is the flush kit:
http://www.jlaquatics.com/phpstore/s..._ID=ro-kpflush
(Although they are "Kent" brand it doesn't really matter what brand you use - they just fit on 1/4" tubing basically, so they will work on other brands of RO/DI units.)