Most desktops have a removable flash chip. For laptops, you can desolder the chip and run magnet wire outside the case. With a little work, you could attach th

Most desktops have a removable flash chip. For laptops, you can desolder the chip and run magnet wire outside the case. With a little work, you could attach the magnet wire to the SD cardreader contacts inside the case. There are also chips with fuses you can blow to make them write once. Somewhere back in my comments is a link to a datasheet. But all of that doesn't really matter, because OEM's don't ship firmware images with checksums or signatures.