As others suggested, very straightforward once I gathered up the courage
to break into the loom. It basically interrupted two of the live
supplies associated with the ignition switch and didn't in fact do
anything unpleasant behind the fusebox, as I first feared.

Still took half a day to expose all the wiring, and half a day to
reassembly the radiator and the rest of the stuff that had to come out.

Fortunately not an engine with an ECU.