I've been doing a little research about it and is fairly simple when it comes to hardware. Actually to add an image to an existing image the only needed thing is to tie both cables together, but both images need to be synced so they match in position inside the display. This is easy acomplished by using a simple piece of hardware: all needed is an small LM1881 to get horizontal and vertical sync and input them inside the STM32F4.
Then the STM32F4 just creates video images with text and lines properly synced to draw horizon, compass and battery, for example, in a video output that is tied to the camera output and transmitter input so overlay is done inside the CF2 for computer-less flight.
Here is a simple hardware implementation example:
The idea would be to get something like this in the display receiver mounted (in my case) in the Devo7e:
Unfortunally I currently lack the hability to translate current existing composite video overlay libraries from arduino to STM and create the empty video with text/graphics onboard the CF2 so I'm throwing here the idea here for the future.
If we manage to get it working it would be the smallest fpv with onboard telemetry without the use of a computer
Fot those interested in my FPV bill:
2gr 120º Camera: http://www.ebay.com/itm/271803570647?_t ... EBIDX%3AIT
5.8Ghz Transmitter: http://www.ebay.com/itm/Boscam-FPV-5-8G ... 1948050797?
5.8Ghz Receiver and DVR: http://www.ebay.com/itm/252003499874?_t ... EBIDX%3AIT
5.8Ghz circular polarization antenna (less image noise): http://www.ebay.com/itm/261978293827?_t ... EBIDX%3AIT
Support for the display to the Devo7e: http://www.ebay.com/itm/231123177859?_t ... EBIDX%3AIT
The total amount is about $170 the more the less, depending on to who buy the items.