Aeroplanes have been living with fire (in their engines) for over 100 years. This COULD be a precedent for permitting (inadvertent) fires in another power source, the batteries, provided it was as well contained as the engine combustion???
Aeroplanes have also been living with a means to cut off the fuel to a burning engine for over 100 years.
If the batteries in question were installed so that they could be isolated, then doused with large quantities of water (paddling pool volumes) and/or ejected from the airframe, then it might be worthy of consideration.
No pilot that I know would be happy with a piece of equipment in an inaccessible area that was a demonstrated fire risk, "contained" or not. What's the point of all the dangerous goods legislation if you carry something similar to a prohibited item around with you all the time?