I just finished a simple telemetry OSD for the Raspberry PI: https://befinitiv.wordpress.com/2015/07/06/telemetry-osd-for-wifibroadcast/
It parses FrSky messages and displays the data as an overlay onto the video. You can see a video of it here: https://www.youtube.com/watch?v=v59qGO08_ck
The OSD works well (but not necessarily) in conjunction with wifibroadcast: https://befinitiv.wordpress.com/wifibroadcast-analog-like-transmission-of-live-video-data/
The OSD is still very basic but I think it is a good start.