You can mount the efi partition, but I don’t think you can usually mount the uefi or bios. I’ve only ever edited vbios, and haven’t done so in quite some time, but I remember needing to clamp the vbios chip. Dunno if motherboards make their bios chips more accessible, but I kinda doubt it.

Some motherboard support starting bios/uefi updates from a booted OS, so there might be a vector to be found there.