The talk here of condensation and humidity coupled with a constant electrcal potential from a battery makes me wonder if the same mechansm might be at play in the 787
Could well be! If your box is not hermetically sealed (and most of the 'internal' electronics boxes are not) then it is required to provide drain holes to prevent condensate entrapment. This means all electronics, wiring and connectors within the box are expected to cope with a humid environment - all pcbs conformally coated and so on.
Every piece of equipment is tested for function under high humidity as part of qualification testing.
If the ELT is intended to be sealed - which it would be the case for equipment with an immersion spec - then it will not be expecting to be damp inside and I guess may not work too well if it is! The post above mentioned a 'wire entrapment' I suppose if Dr Murphy's law is working, that could have been the sole cause, wire trapped preventing case closing and sealing correctly plus insulation damage.....just a thought