Take two raspberry pi's and have one be the hdmi "in" and the other be the hdmi "out" (they could communicate over ethernet). Place this between your TV and your input and inject information on the screen such as notification bubbles.
Edit: Using a hdmi hub would probably be simpler, but not as cool.
Edit2: Using a HDMI splitter and a single raspberry pi run led's around the back of your tv and using the input of the hdmi light up the led's. like http://www.youtube.com/watch?v=LOxKkf8LrbU&list=PLFD09F92CF1DF3FBE, but without having to have a dedicated computer or only working from output coming from the computer.