As written on http://wiki.splitbrain.org/wiki:events_list#html_editform_injection
the event 'HTML_EDITFORM_INJECTION' should be triggered by inc/html.php! But it isn't. At the moment it won't be triggered from anywhere.
It seems to me that the whole event system and its trigger are not very well implemented at this time.
Though it's very useful espacially for plugin development it would be very nice if this could be fixed very soon - thanks!
(maybe there should be an extra entry under Task Type for Events/Trigger)