One thing to remember is that the elt batteries have only a fraction
of the capacity of the main a/c batteries, have a very different
chemistry and wouldn't suffer from the same problems in service.
Such batteries are also often current limited by design. ie: They
would be very unlikely to have enough available energy to burn
through the metal enclosure.
I had to source some long life lithium batteries for a project some
years ago. They were Lithium Thionyl Chloride chemistry, C cell sized
cell, with a high 19.5A/hours rating, but only at very low drain
current. They were rated for 125 C service, with welded stainless steel
outer case. Also found some other Lithium cells during the search that
were rated at over 200C, for applications such as deep oil well data
logging.
I find it hard to belive that these batteries could have caused the
fire, but we just have to wait and see...