Hi Jens,
Thank you for your feedback in this and your other last posts, there are not normal issues and we can learn considerably with your experience. It's the first time that i've heard the sender DTD check in PI. I haven't check it, but have you seen if the Sender SOAP Axis has any proxy characteristic?. Another alternative is like you said to do a java mapping or to deploy a servlet in you PI to take first the message, but it seems to be a bit complicated and difficult to maintain. I remember one servlet that did in my first years with XI 3.0 (with different purpose that your problem) , and it was a really headache .
Regards.