Hi,
Any better ideas?
Just use a "missing pulse detector" with a 555 and a few other bits. The less components you use, the less chance of a component failure in your emergency circuit.
Using a micro like you want, it could get into a loop (if hit by ESD or similar), keeping up the AC even if the TX or RX stops working. Keep it as simple as possible and remember that nothing can be made 100% stable (or emergency kill switches wouldn't be needed at all).
You could use a spare channel for a Dead Mans Handle, which would make it stop if you dropped the TX-box, felt asleep or whatever.
You don't need a sine for an AC relay, a square wave should be fine, but it might ruin your plans that it will hang on DC as well, unless modified with a large pass through capacitor.
If you want to improve the failsafe idea, you should use eg. 2 series connected transistors for controlling the relay, as the typical failure mode for a transistor is shorted. with 2 of them, it would be less likely to get a total shunt (but you'd need to check both transistors on a regular basis).
