(Chris, feel free to move this discussion to a different section of the forum..don't know if you want to do a separate section for these forums or whatever)
I'm looking forward to working with Remzibi on this :-)
Chris, it would be helpful to me to understand what you're goal is for this project. I'll toss out a couple of ideas I've had and some background on related technologies.
This capability could be used to permit a system with an autopilot and FPV that sends telemetry from the aircraft to the groundstation/laptop via the FPV video signal rather than requiring a separate XBee link? This would only be a one-way data link, of course, from the aircraft to the ground.
This would also permit you to move the video overlay function to the ground unit if that was desirable (i.e. send telemetry and the camera video in a single video channel and then do the overlay on the ground rather than in the aircraft which would save some weight/space in the aircraft. This might also address the situation where the autopilot and the OSD both want to connect to the same set of sensors (gps, airspeed, etc).
Here's some background on combined video/data technologies from my own knowledge -- I worked on some of this technology a long time ago -- and wikipedia. There are at least two different services that currently transmit ancillary data in the (non-visible) lines of a standard NTSC or PAL video signal. Those non-visible lines are referred to as the vertical blanking interval (VBI).
In the US, the Closed Captioning service uses line 21 of the VBI to send text subtitles along with the video and audio. It has a data rate of 960 bits per second. Most if not all NTSC television receivers decode the closed captioning and can optionally display the text on the screen overlayed on the video images. You can specify the position on the screen where you want to the text to appear but I believe it is a fixed character set.
In the UK a service called Teletext was developed in the UK and, as I understand it, is/was in use throughout Europe to embed digital data for information services in the PAL video transmissions. This standard supports significantly higher data rates than the Closed Captioning service -- up to 7k bits per second per line used with the potential to use up to 17 or 18 lines in the VBI. I believe that many PAL televisions include hardware to decode and display the teletext data with the ability to overlay it on the main display.
For our purposes, of course, we would like to insert serial data from the autopilot or other airborne device into the transmitted video from the aircraft and then extract that serial data stream from the video receiver on the ground and feed it into the groundstation, laptop, or other hardware. We might be able to use existing Closed Caption or Teletext solutions or just do our own insertion and decoding of data into the video signal using custom hardware/software.
As a next step, I thought I would check out video transmitter/receiver hardware used for FPV to see if the components they use happen to support VBI data insertion/extraction. Any favorite video tx/rx hardware I should look at first?