A "packet writing" UDF file system for CD-RW is currently available.

http://lists.suse.com/archives/packet-writing/
http://w1.894.telia.com/~u89404340/patches/packet/
http://sourceforge.net/project/showf...p?group_id=295

I've successfully installed a patched kernel for UDF CD-RW on Debian.
It should be easy to do the same on Knoppix too.
It's not that fast but it's really useful.

Now, I'm seriously considering any possibility of booting Knoppix from
UDF formatted CD-RW and mount it as UDF file system so that I can
access both read and write.
If this is possible then persistent home should be more handy.

Any suggestion?