Can you please elaborate, possibly with examples?
If I understand you correctly, are you asking for something other than reflection, runtime querying (a la XSLT) and heirarchy traversal (DOM)?
If you think of your "system" as a graph objects that can be traversed; you can require a few messages to be built into a every object, likeconstructors, toString, and generic type-conversion for type-pairs. You can even encode full type description of messages and their arguments, using an schema language XSD, exactly like SOAP does.
Gaaah, I hate underspecified problems that sound interesting.