Handling the XML Prolog inside SOAP

I'm reposting this question as I did not get any response earlier.
 
The SOAP PR spec [1] states a SOAP message must not contain information that
often goes in the XML Prolog such as: document type declarations, processing
instructions or comments before the main document element.
 
A question. What should you do if the XML that you want to transport was
originally a complete XML document (perhaps digitally signed) that contains
this type of information and you want this information to be preserved when
the document reaces the ultimate destination so that it can be used?
 
My reading of the current SOAP spec suggests that this is not possible
unless you Base64 encode it ... or am I missing something.
 
David
PS I realise this may have been already discussed but I missed it.
 
[1] http://www.w3.org/TR/soap12-part1/#soapenv
<http://www.w3.org/TR/soap12-part1/#soapenv>