I have used cowbuilder to set-up a chainroot with ubuntu+1 (essentially minimal ubuntu installation in a sub-folder, which you can "change" into, for example for building packages in a clean environment).
How can I start gnome-session from there? Ideally I want it to appear on a new VT. I hope to achieve poor-man's virtualisation ;-)
You should be able to do this with a combination of startx and /etc/X11/Xsession, as long as you've got the necessary trees bind-mounted (I'd guess that you'll need /dev, /sys, and /proc)
Chroot into your… chroot and run
sudo startx bash
which should give you an X term with a shell. To start a GNOME session you can simply run
/etc/X11/Xsession
from that shell.
You should be able to get this on the VT of your choice; check out the man page for startx for this, and other potentially interesting options.
Note that this doesn't make for a secure chroot - since /proc and /sys and /dev are mounted in the chroot a sufficiently advanced malicious user break out of the chroot with ease.
I have used such a setup a long a time ago. I believe it was just as simple as starting X from the chroot specyfing a different display name:
startx -- :1
Switch to it with: CTRL-ALT-F9
/mounted? with chainroot command you can pretend your/is temporarly somewhere else, which allows you to run "bootstrapped" debian/ubuntu releases.