This seems to be a long self answering thread... :-(

Ok, since no one could/would help me i helped myself. This is not a bug, but a security feature wich
can't be disabled in the config.
I edited the mfsroot file in a hex editor, lol, to add my new line to allow the WAN subnet to be
redirected to a other router, like described in all my previous mails.
Then i had some problems booting m0n0 with the custom created cd again, until i found a iso editing
tool where it was possible to set "allow lowercase filenames".

Works fine now...

If someone could tell me how i can edit,extract,mount or whatever, this file in a regular way or
where i can read about that, i would be happy.