archae86,
Looking at the Thales/Boeing design and the FAA special considerations, it appears that the approach was to assure cell failure would never occur--as it seems self-evident that no serious measures to avert propagation to adjacent cells were employed, nor were serious measures to contain damage to nearby systems.
I'm not aware of any damage to nearby equipment. I agree with Machaca here that apparently the safety systems (or call it hazard mitigation) did their job.
Also, having designed the venting system apparently so that it can prevent damage to nearbt equipment even in case of a thermal runaway and burn-down of a battery, I don't think the engineers just assumed it would "never happen".
The other issue is, of course, that the battery properties did quite obviously not conform to the special conditions set out by the FAA, in particular I cannot see how any Lithium Cobalt (or even Manganese ...) type battery could fulfil No. 2:
(2) Design of the lithium ion batteries must preclude the occurrence of self- sustaining, uncontrolled increases in temperature or pressure.
It is likely that condition no1 ...
(1) Safe cell temperatures and pressures must be maintained during any foreseeable charging or discharging condition and during any failure of the
charging or battery monitoring system not shown to be extremely remote. The lithium ion battery installation must preclude explosion in the event of those failures.
... was agreed to be fulfilled by the FAA, but is now obiously violated. It is hard to see how the described failures of the charger could be seen to be "extremely remote", taken to mean with a probability of less than once in 10 million operating hours, but more frequent than once per billion op-hours. The qualitative specification of "extremely remote" is "not anticipated during the operational life of a single airplane, but may occur a few times in the lifetime of the entire fleet." (paraphrased from CS.25, similar in FAR Part 25.)
Bernd