You can sometimes find the serial lines if you are careful. Otherwise you can use the flashrom to store the output, and read it back out after each failure. It is much easier to just poke around and find the serial if you can, either from schematics (it seems the author has these) or by hand with a lot of patience or board scrying.

because the x280 and x270 are similar enough I didn't need to try very hard to get it to post or boot a live USB to further investigate (effectively acting as a decent template for me to work off of)

The console viewing itself was provided by `cbmem -1`, which I could run via a NixOS live USB with nixpkgs#coreboot-utils