Unfortunately PIX does not support UPnP yet. A request has already been filed for this: CSCdy26037.
Release notes indicate that:
Windows XP Messenger uses Universal Plug and Play (UPnP) for voice and video.
PIX is currently not UPnP aware. This is a request for support for UPnP in PIX.
You can view the request on following link:
http://www.cisco.com/cgi-bin/Support/Bugtool/launch_bugtool.pl
(CCO ID required)
I would recommend to get in touch with Accounts team if you have and present the buisness requirement to them. They may be able to expedite the process.
Regards,
Vibhor.