Currently MC can only push local files via UPNP / DLNA.
I think that JRiver has already considered adding the capability to push remote streams (essentially repushing / proxying the remote stream onwards to a UPNP renderer). This certainly would be doable, although there are also some limitations too, since a) some parts of the UPNP mechanics are optimised for finite length streams, and some renderers might choke on infinite length streams, b) they would have to find a solution to provide some kind of proxy metadata, and c) they would need to rejig the Pause, Play, and Seek commands because of the differences between a local file and a remote stream.
I think this could be a valuable feature request for a new version, although obviously it is not sufficient for MC just to be able to physically push a remote stream, because MC would also need to provide a proper browsing interface that allows the user to easily select which stream they want to push.