If someone is in a position to do this, the chances are they already have physical access, and there are much more worrying things that can be done with that (h

If someone is in a position to do this, the chances are they already have physical access, and there are much more worrying things that can be done with that (hardware keyloggers, etc). The only obvious thing that can be done with this (beyond tampering with firmware) is to backdoor the bootloader/kernel/initramfs/etc, but that can be mitigated with TPM/TXT/tboot (just store your LUKS key in the TPM – any tampering will render it, and therefore your data inaccessible).