Ideally, the DSDT should be fixed within coreboot, but this goes beyond my present abilities.
Not true. Just do the same changes to the corresponding *.asl files in coreboot repo and send the patch to gerrit. Other than a layer of preprocessing, it's exactly the same code as you got from disassembly.
Sorry, I misread you. I thought that you extracted coreboot DSDT from running system then patched it and used as custom DSDT. I'm going to make few experiments on my x220t.