FFMPEG: Make video with non-constant framerate from image filenames
The below answer was provided as a comment on the question by Gyan. I've reposted it as an answer so this question can be marked as solved.




  The image2 reader in ffmpeg can assign timestamps based on the modification time of the file, with nanosecond precision. Not possible on Win, AFAICT. So, on a linux box, you can parse the filename and generate a timestamp and touch the mtime of the file. The absolute timestamps don't matter, just need to make sure the deltas in the mtimes correspond to the desired deltas. Then rename or symlink the images to form a smooth numbered sequence. Once done, read as follows: ffmpeg -f image2 -ts_from_file 2 -i images%d.jpg -vf minterpolate=... – Gyan




This worked for me! I was able to use the touch -d command to set the timestamp on each file based on the filename, then use ffmpeg -f image2 -ts_from_file 2 -i %d.png out.mp4 to create the video. Interpolation ended up being time-prohibitive, but shouldn't really be necessary in my final video.