For a while the developers have been working on getting video exports working with a proper alpha channel, so that we can composite this in premiere or similar.
Turns out this is not really needed!
Here is a step by step guide:
1) Create a long enough video with just a blue screen (RGB: #0000FF) and export it and put it on your phone (I can recommend CX file explorer that can start a FTP server to easily yeet the files over). I put "blue.mp4" in /device/DCIM/RaceChrono
2) Sync the blue video with your racechrono clip. RC will not want to sync it 100% at the start, so you will need to sync it a few frames/seconds/whatever off and then sync it back to the start again.
3) Create your overlay, with huge gauges. You can pretty much fill the entire screen.
4) Export the video.
5) Get it to your computer (again, CX File explorers FTP server is great for this). Easiest is to "move to media folder" and then find it in DCIM/RaceChronoExports
6) Pull the video into premiere pro, and put them on a timeline. Please note that if it is exported in multiple parts it seems like racechrono drops a few frames between the parts, so you might have to space the files out a bit. Having GPS time as a text overlay somewhere really helps to synchronize the videos.
7) Pull multiple copies of that sequence into another sequence, and using crop + motion you can select each area and move it around the screen.
8) And here is the magic: Add "Ultra key" effect, and select the blue color with the dropper. Your video now magically works, even with transparency!
https://i.imgur.com/uGnvAk7.png
