Preseeding a debian installation from the standard installer is as easy as pressing ESC at the right time and pointing it to the url of your preseed file
It is, except when you're using libreboot, and you never pass through that “right time”, because you are skipping the installer's grub.
So, for future reference, here is the right incantation to use at the command line that you get by pressing c at the libreboot menu:
linux (usb0)/install.amd/vmlinuz auto=true url=http://webserver/path/preseed.cfg
simple, once you've found it...
(ok, it took me less than one hour, but I don't want it to take another hour the next time)