How to set custom handler_name metadata for subtitle stream using FFMPEG
Finally figured it out today, and I cannot believe I didn't try this before...Although the tag name is listed as handler_name= in FFMPEG's output, the active tag to use is just handler=, without adding _name.
-metadata:s:s:0 handler="English"
ffmpeg -i "video.mp4" -sub_charenc UTF-8 -i "video.srt" -c:v copy -c:a copy -c:s mov_text -metadata:s:s:0 language=eng -metadata:s:s:0 handler="English" -id3v2_version 3 -write_id3v1 1 "subbed_video.mp4"