Not that I'm aware of. The only phone project I know that's trying to run libre software from the bootloader up is the Neo900 (new internals in a Nokia N900 case). And in the case of the GSM module, if I understand correctly, they're designing a circuit that guarantees you can power it off from the CPU. So basically, their architecture is to not trust the modem, since its running proprietary software.

I would venture and say that none can, but it seems like the GPD Win 2 may be able to run on the mainline kernel with pretty much everything working out of the box, except the touchscreen and the gamepad controls.

No. They can't.