(defun ph/switch-inline-c-comments-to-block (beg end)
"Change whole-line inline C comments between BEG and END to block comments.
If the region contains any lines which are not whole-line inline C comments
then the behavior of this command is undefined."
(narrow-to-region beg end)
This function should cover the first cases (comment delimiter followed by " TODO: ")
(defun insert-todo ()
(insert comment-start " TODO: "))
This one covers the org-mode case:
(defun org-insert-todo ()
(insert "** TODO "))
You could bind them to a key combination ...