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 sshd -D.
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.
Public domain.