To delay mowing the lawn, I've just been studying the wiring diagrams of the DA40 and 42. This thread made me wonder what would happen if the short circuit that was reported in a DA40, in the current occurrences list (200610457), happened in a DA42. The battery relay short circuited, so the electric master had to be switched off, but luckily, the DA40 was on the ground. If this had happened while airborne, after the smoke had cleared, the ECU backup battery should have kept the engine running for up to 20 minutes, and the backup AI should have been OK with its own emergency batteries, but the radios, navaids and GPS would have been unavailable. Not a good time to be mid-channel, but otherwise a situation you could cope with.
The DA42's wiring is more complicated, but it looks as though the situation would be the same, except that each ECU would be powered by the alternator on its engine. Normal operation of the landing gear would not be possible, as there would be no power to the main bus, so it would have to be dropped manually, and there would be no risk of the engines cutting out.
Protecting the ECUs from transients only seems to be necessary when the battery is discharged and the electric master is switched on. It seems to me that the simplest solution would be to put a voltmeter across the battery, and display the output from that on the MFD. Displaying a low battery voltage alert should make most of us reach for the POH.
Can't put off getting the lawn mower out any longer ...