In your original post, I think
Code:
# mkdir -p /lots_of_free_space/source/KNOPPIX
# mkdir -p /lots_of_free_space/master/KNOPPIX
# cd  /lots_of_free_space/source/KNOPPIX
# cp -a /dev .
# cp -a /UNIONFS/bin .
...
can do the job, to really purge the "hidden" packages. But the whole procedure you described to get at least a new and small ISO, seems to be very complicated and not for an inexperienced user.

By the other way, if you first remaster, then purging all not needed packages and the lots of dependencies will be done automatically by "aptitude".

In the scripts of Forester I cannot find a hint, how to purge "hidden" packages by building a new ISO. Perhaps I didn't searched for it enough.