Consider outline-heading-end-regexp in outline-font-lock-keywords.

Example of use case:
1) emacs -Q
2) in *scratch* buffer, type ";;; Hello" in a new line, plus some line breaks
3) eval:
   (setq-local outline-regexp ";;;*")
   (setq-local outline-heading-end-regexp "\n")
   (setq-local outline-minor-mode-highlight 'override)
   (custom-set-faces '(outline-1 ((t :background "red" :extend t))))
   (outline-minor-mode 1)
4) expected: since the outline-1 face has the extend attribute and the
heading regexp ends in a line break, the outline-1 face on ";;; Hello"
should be applied until the line break (extended)

Patch: