So, I've dug up my corebooted t440p and decided to check if it'll work with the battery from my t480, and it did! Well, sort of.
Since coreboot also replaces the embedded controller firmware (mb sometimes they keep blobs of it, idk, but certainly not in case of t440p), we won't get those nasty "battery not supported, pay me" messages even if they've changed the verification since then.
However, I suspect some batteries may be unprepared for the power draw of earlier models. I've tested it on 2 batteries, one was a 22wh → 72wh conversion with BMS built on top of a cheap controller with rather unpleasant feedback from battery repair people; the other one was a more trustworthy 72wh clone powered by bq8050. The latter one worked ootb, while the former somewhat worked: fine in uefi, fine in grub, drop voltage to 0 as soon as the os starts loading → poweroff. If the power supply is plugged in during boot, the battery works fine (may drop voltage again under load, haven't tested it myself).
Soo, basically the use case is that you can try to retrofit the guts of a newer battery into older thinkpads if those run core/libreboot.
A bit of an update here: I decided to do it. Basically, 1st you need to desolder the flex cable, starting with 2 positive wires and not shorting them to other stuff (I haven't tried doing it myself, but it doesn't seem like a good idea)
Then solder everything but positive, isolate, solder positives, isolate. I used hot glue since I'm in the middle of nowhere and too impatient to wait for some more appropriate stuff to be delivered.
Then install the contraption into the case which doesn't fully close now, but it's unnoticeable when plugged in into the laptop.