The thought of a big battery to power everything is crossing my mind at the moment!

Ok, think I'll go back to the simpler approach.
Relay with a 240ac coil that's connected to the primary side of the TX, which disconnects a load dump resistor across the main capacitor when powered (ie resistor connected through the NC contacts).
Then a Returned Energy Dump connected up to each drive. At least I'm assuming having one on each drive is the better option?