While this posting is a little late, I couldn't help offering my ten cents worth.
Write a tiny Java Servlet that does what you ask for in your ?ReadDocument (or ?ReadNote). The syntax would have to be somewhat modified e.g.
http://host/servlet/ReadNote?replica=xxx&unid=xxx or
http://host/servlet/ReadNote?db=db.nsf&unid=xxx
Although a servlet bypasses ordinary database acl security because it runs outside a database scope, you could ensure a some level of security, by having the sevlet only generate the dxl for the note if it has a special, say, $AllowDXL field set to "1", and you could use the session cookie to establish credentials.
The servlet does not have the performance penalty of an agent, since initialization is done on first call.
It shouldn't take more than half a day to write this servlet for anyone familiar with Java/Servlets and XML, and the Domino 6 Designer Help even has info to get you going:
http://www-12.lotus.com/ldd/doc/domino_notes/Rnext/help6_designer.nsf/855dc7fcfd5fec9a85256b870069c0ab/e5b243be743bb49c85256c54004b8696?OpenDocument
Anyone up for the challenge?
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. . . . Write it in half a day yourself usi... (~Bill Quetfooch... 10.Jan.04) 
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