I recently wanted to extend emacs rgrep to search within org files only. Here is what I came up with:
(defun jc/rgrep-org (regexp dir)
"rgrep for org files only"
(interactive
(progn
(grep-compute-defaults)
(let* ((regexp (grep-read-regexp))
(dir (read-directory-name "Base directory: "
nil default-directory t)))
(list regexp dir))))
(rgrep regexp "*.org" dir nil))
Note that in order to get this jc/rgrep-org function to work, I had to study and borrow from the internals of rgrep. In particular, the interactive form was most inspired from studying grep.el.gz.
M-x find-function rgrep for more details.