I finally got around to move my root filesystem to LVM on my NSLU2. This meant that I had to move the root filesystem away from the USB-stick in to my external HD and reconfigure the NSLU2 to boot directly from LVM instead of a simple partition.
The process was quite straight forward. I created a new logical volume on my already existing volume group for root and swap. After creating filesystems I simply copied all the data over to the new partition, updated fstab and ran update-initramfs. The only thing that was different from normal desktop/server hardware was that I had to update the APEX bootloader configuration a bit to allow the USB drives to register for LVM probing.
After sweating for a minute or two while booting the box everything was working fine. It was almost too painless.
Next I need to add the second HD and configure RAID on the HDs. I need to mirror the data that isn't stored somewhere else. Most of the data is backups so I didn't prepare for this initially. Planning ahead always pays off and it tends to bite you when you ignore it. Luckily this is easy to fix.
I'll just cross my fingers that remember to add the second HD before the first one happens to fail...