Well, I now have Lenny on my Mini
Here is what I did:
To start with I had Gentoo installed for about a year and I was getting by but rather frustrated with the emerge stability problems and inability to install basic software like samba etc. My partitions were already set up as described in the Gentoo Wiki.
1. Installed debootstrap using emerge. As always with emerge there were lots of dependencies and problems.
po4a: The build was taking hours and eventually crashing translating Italian binary man pages (it.po). Solved by doing an emerge and when the files are extracted, delete all occurrences of it.po.
dpkg: Could not get this to work so found the source on the web and did a manual install.
debootstrap: Installed using --nodeps when the only missing package was dpkg which was manually installed.
2. Did stage 1 debootstrap directly on the Gentoo LS Mini.
3. Did a second stage debootstrap directly on the Gentoo LS Mini. Also copied everything from /lib/modules into this debootstrap dir. Created an archive of this as described in the wiki and configured my network in /etc/network/interfaces. I set the hostname and /etc/resolv.conf.
4. At this point I could chroot into my debootstrap dir and make sure everything was ok.
5. Opened the LS Mini case and attached the top drive to my Ubuntu PC using USB (the drive next to the ethernet port). Mounted this on my PC as /dev/sdc3 /mnt/root.
6. Removed everything from the root partition (/mnt/root).
7. Extracted the debootstrap fs that I prepared in step 3 as described in the wiki. Local settings were already configured in step 3.
8. I put the drive into the LS Mini and booted with just the top disk. Startup was quick and successful.
9. I put the second disk back in and booted up.
10. I added the second disk to the RAID:
mdadm /dev/md13 -a /dev/sda3
# Then observed in /proc/mdstat that the resync was occuring. This was pretty quick as the partition is only 2GB.
mdadm /dev/md14 -a /dev/sda4
# Then observed in /proc/mdstat that the resync was occuring, this one will quite a while (still running).
Thanks a lot to rpinchbeck for the excellent Wiki and to LaitrKeiows for the gentle "nudge" to give debootstrap a go.