base-config can use some information from the first stage installer.
debian-installer copys its cdebconf database to
/var/log/installer/cdebconf

debconf/priority
mirror/http/proxy
mirror/distribution
debian-installer/keymap
debian-installer/serial-console
debian-installer/country

Future possibilities include:

- using what what d-i uses for its language/locale selection helping set
  defaults for time zone config and for mirror selection
- using mirror selection from d-i, if one was selected there, to
  avoid repeating it in apt-setup
