You can try to use the code of the InfoBarPip class in InfoBarGenerics.py. There you can see how to play a service in the PiP window.
And what to play is defined with eServiceReference... 1 is for DVB, 4097 is gstreamer stuff and so on.
Examples:
sref = "1:0:1...."
ref = eServiceReference(1, 0, sref)
or:
ref = eServiceReference(4097, 0, "/hdd/movie/movie.avi")
or:
ref = eServiceReference(4097, 0, "http://192.168.2.11/movie.mkv")
But you need the plugin gst-plugin-neonhttpsrc for http streams.