Have you tried just hanging your watch in a place in front of you instead? I mean your 920 doesn't have an optical HR (I believe) like the newer models so you still have to use an HRM strap. Just setup a screen with 2 fields for HR and cadence so it is the biggest text size possible.

And that garmin extended display feature is only with other garmin devices I think (like a forerunner to a edge). It won't work like a chromecast or Firestick is what you are asking for.