If it helps to have a more trimmed-down example of what I'm doing wrong, I have a repository here with a mostly empty configuration. build.sh kicks of the build. I am actually performing it in a virtualbox vm inside vagrant. If you have vagrant/virtualbox, you can run ./build_via_vagrant.sh -f to kick out the ISO.
Installation works perfectly in EFI, but as I said not in BIOS.
-SG