how peculiar. I didn’t actually test it, I just assumed that:
You can set the value for any id3v2 frame by using '--' and then frame id
id3v2 --TIT3 "Monkey!" file.mp3
meant that you could set the value for any id3v2 frame by using — and then frame id.
My mistake. (?!?!?!)
So I guess I have no idea what you would use to set that flag. I guess all mine are in iTunes, and I use that. How strange.