Hoo boy. Things not to do: run make-kpkg on 2.4.0test12 and then dpkg -i the resulting package without realizing that this kind of writes a new symlink to /vmlinuz. I wound up at one point having two lilo labels pointing to a 2.4 kernel that would not boot, and the one remaining one (thank god) pointing to a 2.2.16 kernel, a failed attempt at getting ip masquerading to work over the summer. But it would boot and let me fuck with lilo enough to get my 2.2.13 kernel working again.
Still haven't found a 2.4.0 configuration that boots correctly, though, rrgh.