And Plamen, thank you.
After 24 hours of staring at this post, DK has figured out Phase II. She just gets a WaterWatcher and patches it into the circuit, plugs her master valve solenoid into it, and voila.
She was sipping joe from the Bubba Mug, and achieving alpha wave status just now, and it hit her.
While it is a proprietary component, it will likely be nearly as cost effective as buying parts & pieces (relay, power supply, outlet, project box, etc.), and it will definitely be more time efficient and space efficient to go with ready made in this case.
Probably could have just used WaterWatcher as the main detector and alarm unit, too, but we already have ones so will just patch it in. Need to see sensitivity on it to see if it can read the circuit properly...