Chris - you are right, the solenoid is a good backup. I just wouldn't suggest using one as the primary flow control, especially if you connect it to an RO unit, unless you use some sort of backup with it.
I will probably end up getting a solenoid to use as a backup to my system. Mine uses a powerhead connected to a Lifereef float switch. I have a siphon break in the line, just at the top of my trash can, otherwise the topoff would continue to drip after the powerhead shut off due to the siphon (learned that one the hard way - fortunately I caught it before a flood). Eventually I want to get another float switch connected to a solenoid, so if the siphon-break hole ever clogs, when the sump water reaches a certain level it will shut off the solenoid.
Of course, another option would be to just get a Tunze osmolator. I'm pretty sure it incorporates all of those backups I mentioned, and their float switches have no moving parts so they are less likely to get stuck or fail.