For a little bit I was using a Nexus 10 as a secondary display - I used Google Chrome's remote desktop to view a separate X session, with Synergy for sharing my input devices between them (which is now paid for?!). Worked great, except you couldn't see the mouse cursor on the Nexus... :( Ultimately it was more annoying than helpful. (Edit: just realised that's broadcasting in the opposite direction to what you're asking for)