My understanding is when a cellphone has weak service it boosts the signal to try and reach a tower. In my experience, it feels like it drains the battery incredibly fast. Are there OSes or phones that accommodate this?
That's correct - if the signal is weak the battery is drained quickly. I'm not aware of any OS or phone which can mitigate that - is there any other way than to give up on the connection?
An extra phone or device as a hotspot
If you're battery constrained and not expecting a signal most of the day I imagine you could put the radio to sleep for 5-30m. I kinda understand a weak signal getting boosted and draining a battery, but when there's none at all? It just seems like a use case they didn't care enough to address. Since it sounds like this is the primary use case, I figured someone might have tried.
True. I notice it when out camping in the mountains where there is no reception. On airplane mode the battery can easily last 2 to 3 days max but if you leave airplane mode off, it will die within 1 day.