A custom mapping with:
messageCache.setValue('messageHash', qie.getMD5Hash(message.getNode('/')));
messageCache.setValue('messageSize', message.getNode('/').length());
And add this to the SOAP:
<rim:Slot name="hash">
<rim:ValueList>
<rim:Value>{mc:messageHash}</rim:Value>
</rim:ValueList>
</rim:Slot>
<rim:Slot name="size">
<rim:ValueList>
<rim:Value>{mc:messageSize}</rim:Value>
</rim:ValueList>
</rim:Slot>