How do I have Heads boot from USB by default?

I’m running Qubes with a detached boot partition and tried to set up Heads. It seems that the BIOS doesn’t let you proceed with functions such as adding a GPG key if it doesn’t see a boot partition on the internal drive. Is there any way to fix this and have Heads initialize USB drives and boot into them by default? If this is not supported then are there any workarounds? Can I edit the scripts that Heads uses to change this?