> coupled with some physical BIOS chip reader. And the chip reader needs to be cheap enough for us to assemble ourselves; we can't really trust some company to make it for us, because the company could be coerced into subverting it or it could be subverted in transit after we order it. So it seems like we need an open source blueprint of a BIOS chip reader that's cheap and easy enough for anyone to make themselves. (A tall order, to be sure.)
BIOS chips these days are almost invariably SPI serial interface, which means it is not difficult to make a bare-minimal "manual" one with a battery, a couple of LEDs, resistors, and switches. Just key in a read command by hand, then read out each bit as you pulse the clock line. Pretty hard to subvert something as simple as that, but the problem is how long it will take, so maybe you could do this just to verify the bits that comprise a "root of trust" and not the entire BIOS image.
BIOS chips these days are almost invariably SPI serial interface, which means it is not difficult to make a bare-minimal "manual" one with a battery, a couple of LEDs, resistors, and switches. Just key in a read command by hand, then read out each bit as you pulse the clock line. Pretty hard to subvert something as simple as that, but the problem is how long it will take, so maybe you could do this just to verify the bits that comprise a "root of trust" and not the entire BIOS image.