I've been looking also. I found topdose.net today and sent them the message below (heard they will also do custom work), waiting for an answer.
They have a unit designed to do just what your after (model DDB) and there prices seem reasonable compared to others I've found.
rm
"I'd like to use your system to control my RO unit through a water solenoid.
Can your controller handle direct connection to a solenoid (worry is the inductance will fry the controller or float switch if a relay is not used in your controller to switch the output).
Also, my house water pressure is low. My RO is rated for ~50gpd but only produces 20gpd. i.e. I need a timer with a longer countdown to shutoff. Can your controller be modified to increase the time?
It would be nice to select from minutes to several hours and also be able to run in timer mode only (thinking double duty for your controller by using it to refill my 30gal water storage tanks via the same RO but by timer only, this only needs to be done occasionally and I could get by with manual topoff of the sump when using it to fill the storage tank)"