Thanks for the reply. Hmmm... can't see to view your GIF. But in any case, I got it to work when I kept the selection marquee turned on (instead of pressing Esc to deselect after copying it to frame 12) and in that case it did work.
EDIT: I'm trying again today and it works now exactly as advertised and I can't seem to reproduce my earlier problem... Will report again if I bump into it, but maybe I didn't reboot Aseprite after loading your script. Not sure if that makes sense but...