Put modified xml back into the message?

Put modified xml back into the message?

I'm using CXF to send messages with SOAP over JMS.
I'm trying to write a CXF Interceptor in the POST_MARSHALL phase. I want
to change some attributes when the xml is generated.
I know i can get the content from the message via
message.getContent(java.io.Writer.class).
This happens to be in the form of JMSConduit$1. Which - I think - is a
StringWriter (if I debug my code I can see a buf field).
I can get the xml in String format and make my changes, but the problems
is putting it back in the message.
I can not change the JMSConduit$1 to something else, otherwise CXF won't
send it to the JMS Endpoint. (it must be a JMSConduit).
I can't find a way to put the modified xml back in a JMSConduit, which i
can get through
message.getExchange().getConduit();
So, how can I put my modified xml back into the message/JMSConduit?