So the computer boots up and passes it to the bootloader
And here does the plan fail. There is simply no way to do that. Bootloaders expect the vga being in 80x25 text mode, period.
Doesn't Apple's computers do that? I don't know.
Yes, they do.
Would it be possible to boot in a resolution any more modern than the old IBM PC BIOS of 1980?
As long as we are using the IBM PC BIOS interfaces if 1980 (which seabios is all about): No way.
With EFI (which apple uses btw) it's another story ...