After doing some more experimentation myself, I have come to the conclusion that a Knoppix-adapted version of the debootstrap utility (something like "knxbootstrap") would be a good tool. With Debian 6.0.5, debootstrap creates a bare bones install of size ca 200MB. It's probably not worthwhile to go any lower for general use. I haven't looked into any details about the debootstrap utility, so I have no idea what it could take.