Non-standard installation (installing Linux from Linux)

Ubuntu has an article on Installing Ubuntu from a Unix/Linux System , using debootstrap to do a "cross-install." It won't be a perfect fit to what you're doing, but it should get you on your way.

For all Debian based distribution (like is Ubuntu, see http://wiki.debian.org/Derivatives), you should really look at the debootstrap tool which is the correct solution to this problem.

Look at some tutorials on the net because you will need to debootstrap and then to customise it with chroot.

Anyway this installation method is totally supported and one of the best ways to do it, so there should be no problem (I think the Debian and Ubuntu installer CD are using it).