Well, there's this:
Backing Up via Rsync - WebOS Internals
Another option would be just good old tar. But the one on the device is weird...so you'll need gnutar, which can be found here:
http://ipkg.nslu2-linux.org/feeds/op...1.22-2_arm.ipk
after you install that, you could use gnutar to backup the filesystem to somewhere else, like the /media/internal space. So something like this, I think would work:
gnutar -cvzf /media/internal/bigbackup.tgz / --exclude /media --exclude /sys --exclude /proc --exclude /dev
Obviously -z is going to be slow because you're running it all through gzip, and that's a bit pokey on that little CPU. I think that syntax will exclude those four directories, which I think are full of stuff that can't be backed up. /sys is a particularly bad one to descend into--touching some of the files in there causes the phone to reboot.
Someone please school me on my commandline; I'm sure it's not as sophisticated as it could be.