EEngr
There is no way anything other than a Honeywell part could be used and maintain certification I would hope?
As you point out, the polyfuse (or whatever) must be part of the battery pack to be any use, hence my thought about it being in close thermal contact with the cells - perhaps deliberately in the hope that it would shut things down if the cells got to hot? Wouldn't be any use in a thermal runaway situation though.