Well it seems it is a non-issue for Boeing because they took into account a possible battery fire and say the airplane can stay aloft with a full-blown battery fire. presumably for the full ETOPS time frame.
I'd be very surprised if that was the case. The electrical and chemical energy stored in one of those battery packs is significant. (24V/65Ah was being quoted for the APU?) Normal hold fire suppressant (halon) would be effective against flames coming out of a pack, while the concentration was high enough, but would do very little to stop what's going on internally. That's why the advice for a lithium battery fire is halon first, then lots of water to quench it - I don't think the 787 has water sprinklers in the hold.
Just one AA cell that goes bad can be quite spectacular - a pack equivalent to a stack of car batteries...?
It took a team of professional fire-fighters 40mins to put the 787 in Boston out and that was on the ground with specialist equipment.