Thanks Peter and naj.
Yes, I have seen those fitted on some cars here, but only on (English) cars where the battery is located near the firewall.
I am reluctant to run the main battery cable from the front and up to the firewall and back again, but I expect the voltage drop will be acceptable.
I am trying to think of a good design to actuate such a switch fitted near the battery, using a bonnet release-type setup.