Looks like it *is* available as an installable Windows service. But I don't see anything on the Chorale page about the functionality I'm curious about. I only see it talking about how it can serve Rio Receivers.

I was looking for something that's the opposite of a Receiver-server emulator. I'm looking for a middleman program that will sit on the network and process UPNP media requests, forward them on to the Central, and deliver the resulting content back to the requestor.

Your post seemed to indicate that you understood that's what I was looking for, so clearly you've got inside knowledge about development of Chorale. Chorale's sourceforge page isn't very helpful to me though: it keeps saying "no matches found" when I try to browse for feature additions in its tracker. Are you involved with the Chorale development, and can you give me more information about how this is configured and how it works?