Thanks to everyone who was involved!
For some reason, the exit code is 2 when running on-demand.
metalog is included in main tree. gdm has been included in gnome3 for a while now, so I don't see reason to keep it around any longer. preload is outdated and no longer maintained, see bug https://bugs.gentoo.org/show_bug.cgi?id=483154
Thanks to Markus Rathgeb for noticing this.