I'm familiar with the process of exporting a single subtitle image at a specific time out as a png overlay:
ffmpeg -ss 00:17:11.88 -f lavfi -i "email@example.com:size=1920x1080,format=rgba,subtitles=subfile.ass:alpha=1" -frames:v 1 70.png
I'm searching for a method of converting the subfile into a complete image/movie overlay. The intent is to use it as an overlay in a video editor that doesn't have subfile support. So it doesn't need to be png, but something that enables the alpha channel. I'd want something with small filesize, willing to go lossey quality.