Power-wise, this unit is completely self-contained, self-testing and with a ten year battery life.
The only electrical integration with the aircraft's systems would seem to be this connection to a flight deck 'test switch'.
Are all Honeywell ELTs connected to the flight deck or just those fitted to the 787?
I am struggling to see how a low voltage/data connection can cause a fire, even when incorrectly connected. And the unit itself in a quiescent state is unlikely to draw more than a few milliamps from the onboard battery, not unlike an electric clock battery which can tick for several years on a single AA 1.5 volt cell.
I can only imagine that if the beacon is the cause of the fire, it somehow suffered some mechanical damage either inflight or during manufacture and installation.