Thank you. The power bank may work as a hardware solution sometimes, though even in a pocket, any battery can fail if the outside temps are low enough (as they often are in winter at higher latitudes).

I essentially was wondering if the heat a phone produces during higher-load operation might be repurposed to create an entirely software solution to the problem of a battery dropping below functional temperature. Your answer suggests it can. Perhaps this could also be balanced by some kind of dynamic heat-dispersion design to account for the converse issue of battery temperature getting too high. But that's encouraging - if modern designs intentionally direct heat away from the battery, that suggests the basic software-only idea might be possible in principle, and maybe in practice - in any existing phone that has poor hardware design in that respect. Components under load increasing battery temperature must be a valid effect, to necessitate designing around it - so using this "feature" to conversely keep battery temperature higher than its operating minimum may be a valid proposition.

Extreme cold situations can also sometimes be an emergency, so it's not necessarily a trivial issue. Granting phones a kind of rugged-cold mode to prevent shutdowns, via a software update that makes simple power management choices dependent on outside-temperatures (phone getting too cold: crunch numbers and activate display), could potentially save lives.