I have a feeling that starting from debootstrap might provide a simpler and safer way, using the Debian live tools where they fit in. I went by a route similar to the one used here myself some years ago, making a 64 bits version of Knoppix 6.4.4. But then I had to modify minirt.gz. IMHO, the most important part of the current approach is handling all the Knoppix goodies with Debian package tools. That might also be a stepping stone towards a reproducible 64 bits version. The case for knoppifying Debian live may have become stronger the last year.