Here is a demo:
Please ignore the Power number as it’s derived from wheel speed in a way which is approximately correct if I were riding on my turbo trainer but is completely bogus on this 4%-average incline.
Technologically it was quite interesting to build. I render the stats onto a bright green background in real-time along with ambient audio from the iPhone microphone. I can then import the video I rendered into FCPX, use the audio track to synchronise it with the GoPro footage and drop a chroma-key effect onto the stats video to make the green background transparent.
Since I recorded this I have removed the bogus power number and added altitude. As and when my power-meter arrives I will add the real power number in.