Normally-Open Kill-Switches & PAMCO

[osteoderm]

Way back when, Porsche used a relay which had a 'ratchet' type mechanism in it to run the high/low beams on the 914's etc.

I'm familiar with bi-stable latching relays as you're describing, which are widely available once you know what you're looking for. They are slightly more mechanically complex, larger, and subject to vibration; automotive-application latching relays need to be carefully chosen with this in mind.

So if you go with this, if you press the kill switch you'd open the circuit to the ignition, thus cutting power. Press the switch once again, power is restored.

In this case, I wanted it to be somewhat difficult to "un-kill" the killswitch, or at least require some additional step so that ignition power could not be accidentally reinstated once the momentary button had been pressed. This both for safety in an emergency situation, and to prevent the PAMCO from remaining powered after the engine had been stopped.