So I restarted the system and set about clearing out the junk:
- ran sudo apt-get clean
- emptied the firefox cache
- deleted banshee's database (easy enough to restore again)
- emptied the wastebin
...and ran fsarchiver again. Just clearing those out shaved a couple of gigabytes from the archive; now it's small enough to fit on a DVD. Are there any other places I could clear files out from? I'm looking to be able to restore the system with the applications and configuration I have now, but basically empty of data. Ideally I should have done this after installing everything I wanted, but before starting to *use* the computer.
As I said there were a lot of cached thumbnails and fonts flying through the list in fsarchiver, but I don't know enough about where these are stored to go deleting things.