Security theatre. When was the last time you saw a piece of boot time malware? I remember viruses that would sometimes find their way onto floppy disks, so if you left the floppy in when switching on, your machine could be infected that way.... my, was a long time ago.
Its trying to solve a problem that doesn't really exist. There are much more valuable and interesting attack vectors for malware writers these days.
Actually the problem of boot time malware was solved in the DOS day with a BIOS setting that prevented to write the first sectors of the hard disc An MD5 checksum of the MBR and the boot sectors saved in a read only memory shoud suffice. I think a jumper to enable flashing the BIOS and this checksum area kill easily the unwanted tampering of the boot sectors.
Save yourself! Reboot in 5 seconds!