I don't really see this being something we specifically need to work on. Realistically, how it would function is a minimize to tray on close instead of actually closing. That would keep the MPRIS widget around in Raven. So to do that, you would need to write a plugin for rhythmbox that supports:
- Having a tray icon.
- Having options to minimize instead of close as well as minimize to tray.
That being said, you can't minimize a Gtk.Window "into" another Gtk.Window.
