I've had a quick look at the manual, downloaded from the Pinnacle web site.
The Showcenter is served by its own media libary server software that runs on a PC, where you import and catalogue your collection. There is a vague reference in the manual to turning on the UPnP service although it only comes under the secion where it describes setting up the registration for the Rhapsody service and reading between the lines it looks as if there is a separate Rhapsody application that must be running and it's this that acts as a UPnP Server, which the Pinnacle Media Manager connects to. There is also a screenshot of the Showcenter itself where there is a reference to "UPnP AV", but other than that there is no mention of UPnP - the Pinnacle Media Manager just comes under a heading of "Media Servers".
I've had a look at dlna.org/products where you can search for products that are UPnP/DLNA certified but I can't find any reference to Pinnacle or the Showcenter products.
Basically, there doesn't appear to be any supporting evidence that you can use a standard UPnP server such as JRiver to serve the Showcenter instead of its own Media Manager program. The only evidence can come from people who have tried it, but perhaps no-one has indeed tried it. If you've got access to a Showcenter and also a trial version of JRiver, then you can try it and report the results.
Why would you want to use JRiver MC with the Showcenter instead of Pinnacle's own Media Manager? Well, we know that JR MC is far more powerful! But is there any particular reason?
