Hi Michael,
If t400 stock bios structure is similar, you might try restoring it using this procedure: http://thinkwiki.de/UEFI_BIOS_T420_BIOS_Structure It worked for me to revert to stock bios from coreboot on a t420. As you write only bios region, you don' t need the old backup of your chip. In fact, I flashed a version of the stock bios that was different from the one I had before coreboot.
Regards, Florian