The logic itself isn't that complex, we just need some further checks in the 
code. A usage example from reallife:

pvcreate /dev/cciss/c0d0p3
vgcreate avl01_vg /dev/cciss/c0d0p3
lvcreate -n root_lv -L8G avl01_vg
vim /etc/debootstrap/config # set /dev/mapper/avl01_vg-root_lv as target
grml-debootstrap
mkfs.ext3 /dev/cciss/c0d0p1
tune2fs -c 0 -i 0 /dev/cciss/c0d0p1
mount /dev/cciss/c0d0p1 /mnt/test
mount /dev/mapper/avl01_vg-root_lv /mnt/grml
mkdir /mnt/test/boot
cp -a /mnt/grml/boot/* /mnt/test/boot/
grub-install --root-directory=/mnt/test /dev/cciss/c0d0
cd /mnt/grml ; mkdir boot.part ; ln -s boot.part/boot boot
umount /mnt/grml
umount /mnt/test