This isn't even my final form! (Because it isn't all that safe; it still depends on the same FS everything else is using. A proper solution would come with kexec stuff.) But at least it should prevent me from doing anything too stupid.
Tests for the volume down key; if it's found, enables USB (method borrowed from
ramrod from the ramdisk; much nicer than the old one!) and execs
Stick the binary in /etc/rc.d (make sure it's alphabetically first), then you
can hold volume down on boot to go straight from the jailbreak executable to
SSH over USB. Run
killall sshd to continue boot.