neat idea. This code does exactly what I need for the completion for a whole tag query. It should be possible to integrate that into completion.
(defun insert-emails-from-tags (tags)
(interactive "sTags: ")
(insert
(save-window-excursion
(find-file "
contacts-bbdb.org")
(mapconcat
'identity
(let ((todo-only nil))
(org-scan-tags
(lambda ()
(org-entry-get (point) "EMAIL")) ; action
(cdr (org-make-tags-matcher tags)) ; matcher
nil)) ", "))))
It should be possible to integrate this into the org-contacts database to get this with completion. I will look into this later today. Thanks for the ideas.