Does anyone know if there a reason they would have to say that (rather that "turn them all off now") for types other than the one they are investigating? I can't think of one unless they suspect some 'unfortunate' interaction between an ELT battery issue and the 787 systems that wouldn't affect other aircraft types?
For 787 case it is already clear that "bad things" can happen if the ELT gets hot.
For others types it is an open question if the same scale event would have breached the hull. Still not something that one would want in flight though.
Also on protection of battery by fuse, from the spec sheet the unit is rated at 5W at 406Mhz, plus lesser at the lower frequencies.
This translates to expected power in use in the 8 to 12 watt range, plenty to get a wire hot enough to cause trouble.
Could be higher, 8 watts would actually be very impressive effeciency figure.