Well, it’s been a while. I did some more experimentation with this in the months since.
First, I made a “solo compilation” video with the “old” method of screen-capturing the Guitar Pro tab as it was scrolling:
Then, I had a go at the method by Troy and @CyborgCutie which is probably the best / most flexible for my original objective, because you have this long png image and you can adjust its scrolling speed, zoom level, etc. by using a video editing software like Final Cut Pro.
In this first application, I exported bars of different widths in the png, so I had to sync each individual bar manually:
Then, I tried to find a method to reliably export a png image with bars of equal lengths from guitar pro 8. Not sure if it’s a bug or I missed something, but while GP8 has the option to have bars of the same size, the png export always reverted to unequal bars when exporting. So I came up with a hack: have a second tab filled with 16th-note pauses, which would enforce the bar size for the upper tab:
Then, I can get away with fewer syncpoints (ideally just one at the start, one at the end), and I can crop the bottom tab in the video editor so you don’t see it:
Now, for the last one, the same trick did not work, since I had some key changes that once again made certain bars have a different size (even with the “16th-pauses” trick). So, unless GP8 fixes the (suspected) png export bug (where you can’t enforce equal bar lengths in the exported image), we are back to having to manually set various sync points to make sure the scrolling is roughly correct around the key changes:
TLDR: I think @JakeEstner was right that at the end of the day all this may not be worth it in a world where SoundSlice exists 
However, I came to the conclusion that @JakeEstner was right all along — this is cool and all but it might not be worth the effort, versus just creating a “slice” on soundslice (where people also have the option of turning tabs on and off as needed).