using the cow file to store changes of the /KNOPPIX filesystem
would be really cool. This would mean, that Knoppix could be
used like a hd installation with every file being writeable!
I've had this idea, too, but this can't be done without hacking
the kernel. At least it seems to me, that it could be done.

At the moment the /KNOPPIX fs is mounted read-only inside the
UML image just like in the ramdisk. And as in the ramdisk only
a few files can be modified, the rest are symbolic links to
/KNOPPIX. One benefit is that the cow-file can be stored somewhere,
so you have a "persistent ramdisk".