Good news: The height editor I'm working on just had a major breakthrough! It successfully extracted the heights list from the GT6TED file and ran it through a height randomizer which changed the elevation of each height point by up to 0.1 meters, and then inserted the modified heights list back to the file and patched everything together again.
I then uploaded the modified file to GT6 and the track works! The track has mostly the same elevation as before, but there are now some random bumps here and there, kind of like the Mulsanne Straight at La Sarthe.
Here is a link to the track if you want to try it out:
https://www.gran-turismo.com/gb/gt6/user/#!/friend/GT_85_Andersson/course/2323757/
The editor itself is not ready to be shared yet though, but hopefully I can have something ready to be shared before the end of June.
Edit: Refined the noise script and applied it to another track, together with a couple of random elevation changes:
https://www.gran-turismo.com/gb/gt6/user/#!/friend/eran0004/course/2324400/
Also, here's how far I've come on the user interface
It's a graph plotting the elevation, slope and delta slope of the track. The idea is that you'll be able to zoom in and edit the elevation directly in the graph. Should be doable. Somehow...
View attachment 556957