Multistrap is not a DebianInstaller environment, it does not run udebs and debconf pre-seeding using d-i prefixes cannot work. It is possible to pre-seed debconf with other values outside d-i.

To get the data for a preseed file, follow the instructions for standard Debian installer preseed files but be very selective about which settings are included. http://www.hands.com/d-i/

Create a text file containing the seeds and pass that file to multistrap in the configuration file:

[General]
debconfseed=files/preseed

The preseed file will be copied into /tmp/preseeds/ in the rootfs and a suitable configscript can then push those settings into the debconf database by specfying the configscript option, in the same section as debconfseed:

[General]
debconfseed=files/preseed
configscript=files/configscript.sh

(Don't repeat sections, add these options to your existing General section.)

Then, once the rootfs has been unpacked, use configscript.sh to configure the packages:

chroot /path/ /bin/bash
./configscript.sh

Example configscript.sh:

export DEBIAN_FRONTEND=noninteractive DEBCONF_NONINTERACTIVE_SEEN=true
export LC_ALL=C LANGUAGE=C LANG=C
/var/lib/dpkg/info/dash.preinst install
if [ -d /tmp/preseeds/ ]; then
        for file in `ls -1 /tmp/preseeds/*`; do
        debconf-set-selections $file
        done
fi
dpkg --configure -a
mount proc -t proc /proc
dpkg --configure -a
umount proc