Switching RCA composite video, under uC control - circuit ideas & suggestions
Use a high-frequency relay such as http://octopart.com/g6y-1-dc5-omron-697923

Use a video switch IC. http://www.mouser.com/Search/Refine.aspx?FS=TRUE&amp;N=12002272+1323043

Use this instead and just overlay the text on top of the video. http://nootropicdesign.com/ve/

If you don't want it to look like the 80's you could try the HDMI-compatible neTV, but I understand you might not be able to do anything about the really-old-tvs or underlying video. http://www.adafruit.com/category/46