Why not a closed-when-off solenoid? As in, power goes to 'noid, 'noid opens, water flows. Power outage occurs, 'noid closes, flow stops. Only issue here would be cost of a suitably sized 'noid. It's definitely something I'll be looking into, when I start planning my own in-wall...
Aside from this, are you using Durso/Stand-pipes? By design, these units will break a siphon once the water drops to the level of the open end of the Durso/Stand...