A linux kernel with unionmount support
make-kpkg --rootcmd fakeroot --initrd --append-to-version -um-i686 kernel-image kernel-headers
Unfortunately kernel-package and initramfs-tools package maintainers can't agree on a way to tell initramfs-tools to generate an initramfs so you will need to generate one in a hook (put in chroot_local-hooks). Also cross-building i686 on amd64 seems broken.
Mount command with unionmount support
-k um-i686+ \
--bootappend-live union=unionmount \
--packages "httpfs2 less open-iscsi links2" \
--linux-packages linux-image-2.6.36-rc5 \