One last detour: Had to do more work than I really want to at this stage, to make the secret-project pick a good disk to use. Hardcoding a disk device was not working reliably enough even for a demo. Ended up with some fairly sophisticated heuristics to pick the right disk, taking disk size and media into account.
Then finally got on with grub installation to the target disk. Installing grub to a disk from a chroot is a fiddley process hard to get right. But, I remembered writing similar code before; propellor installs grub to a disk image from a chroot. So I generalized that special-purpose code to something the secret-project can also use.
It was a very rainy day, and rain fade on the satellite internet prevented me from testing it quickly. There were some dumb bugs. But at 11:30 pm, it Just Worked! Well, at least the target booted. /etc/fstab is not 100% right.