>> What could be possible, however, is to add UTF-8 characters as overlays >> on top of standard bullets. I think "org-bullet" does something like >> that.
This seems to work:
(font-lock-add-keywords
'org-mode
'(("^[[:space:]]*\\(-\\) "
0 (prog1 () (compose-region (match-beginning 1) (match-end 1) "—")))))
