If your threat model is "has access to the system before first boot" you are fucked on anything that isn't locked down to only the manufacturer.
UEFI Secure Boot is also just not a meaningful countermeasure to anyone with even a moderate paranoia level anyway, so it's all just goofing around at this point from a security standpoint. All of these "add more nag screens for freedom" measures like the grandparent post and yours don't really seem useful to me, though.