Ok, so I got this up and running.
Bought this valve: US. Solid SKU JFMSV00027 2 wire auto return stainless ball valve, 1/4" NPT.
These adapters: PRTFITMC14M14Q10PK 1/4" Male NPT to quick connect 1/4" hose
A 12vdc 5amp power supply I had laying around
Float Sensor model DP5200
and a tank mounted sensor holder from Printed Reefing Solutions.
Total cost, oh, about $40.
A bit of speaker wire, a few shrink fit tubes, a couple spring terminals, and presto:
Ball valve is after a manual 1/4" valve, and before a float valve, directly on the line from my RO/DI system.
When energized, ball valve opens. I've got it plugged into my Apex.
When power fails, ball valve closes.
When float valve rises, breaking connection, ball valve closes. Float is in my sump, an inch higher than normal water level.
So... My float valve actuated ATO, as long as the float valve is working, runs on the old fashioned idea of a mechanical float valve. Ball valve stays open all the time. I'm considering setting the Apex up so that it only opens the valve, say, 20 minutes, every 3 hours? Something to keep my RO/DI system from constantly starting and stopping... Haven't made up my mind on that one yet.
If the float valve were to get stuck open, which they all do, eventually... water level in the sump compartment rises an inch... probably 1.5 gallons. Float valve trips, ball valve closes, water stops flowing.
I've got a 2nd high level sensor, attached via breakout box to my Apex. If all of the above fails, the high level sensor on the Apex will kill the power, causing the ball valve to close. It also sends me an email alert.
The only way I see this failing is if the ball valve fails OPEN, and the float valve fails OPEN, and my home internet is down.
Oh, and there's a battery operated water alarm on the floor under the sump
I think I'm about as safe as a direct RO/DI ATO system can make me.