Hi,
I just wanted to let you know that I have done something similar for myself some time ago.
I wanted to use ReelSteadyGo so I had to fabricate the GPMF track into the mp4 container.

I was successfully able to do this, by converting the blackbox gyro data into GPMF format and RSTG was fooled to accept it.

I was thinking about packaging all up as a project and publish it but didn't have the time and motivation to do it. In my case I was doing it by a bunch of standalone python scripts that require somewhat low lever understanding of how things work.
The main reason I don't feel strong motivation to make it public is because even if it is packaged in nice wizard-like interface there's still considerable manual work to be done which I guess will put off most average users.

Probably the most tricky part is getting the synchronization between the video and the blackbox data right. Of course, this is done with Blackbox explorer, but in my case it seems that the clocks of the video and FC are running at slightly different rate. For example if I sync the video at the beginning of the BB log then at the end it has already drifted. And vice versa - if I sync at the end, then the at the beginning it is incorret. That's why I had to sync at the beginning, write down some numbers, then sync at the end and write down again. Then these both sync points are used in the script to interpolate. It works, of course, but all this is getting quite laborsome for the normal user. Also the syncing in BB explorer must be done quite accurately, which requires some skills too.

In the end run, everything works but it is a hassle.

If you are interested I can share with you my work and you can use it in any way you like. Honestly, I don't know if I'm ever going to work on this on my own to make it public.

Here's an example. It is shot with Runcam 3, integrated the gyro data and run through RSTG. The results are plausible overall. I suspect that it might never be quite perfect, because if the camera is softmounted the microvibrations can quite easily be out of phase with these of the FC. For this reason I doubt that propwash can be properly eliminated but I guess in some cases it will work fine.
https://www.youtube.com/watch?v=CYlGe_PhRLg

Best~