Attached is a patch to outline.el defining a new command
`outline-narrow-to-subtree' (bound to `C-c @ C-s').

Let me know if this is okay to apply this patch: before 
Org 7.7, people could use `org-narrow-to-subtree' in buffers
with outline-mode or outline-minor-mode, so this patch can
be considered a fix rather than a new functionality.  But 
maybe it's borderline.

Thanks,