Admin UI pluggability

[Ryan Ernst]

I would like to add some custom pages to the core menu for my setup,
replace some existing (like ping) and also remove some others (like data
import).  From what I can tell, the existing hooks are very limited (like
admin extra that appears in overview for the core).  I've searched through
JIRA for any issues regarding this, but can't find anything.  Any thoughts
on how this could be done?  Can we make the admin UI more pluggable?