One should be able to use PulseAudio and select a different sink than default (localhost) on the N900's PulseAudio daemon. In GNOME, the PulseAudio applet allows to select different server (the PA daemon to control), sink (ie. target, output), and source (ie. input). UPnP-AV and RTP should work as well. But the issue is then the playlists and database of files. In my case, because I use rhythmbox, I can use SSH client + rhythmbox-client to control.