symbol-overlay-mc is a tiny Emacs package enabling multiple cursors on symbol overlays. It was born out of a small itch I wanted to scratch. Wrote how I went about it here. The package lived in my config until Peter Oliver kindly volunteered to bring to MELPA.
A sample config, using M-a as a key binding:
(use-package symbol-overlay-mc
:ensure t
:bind (("M-a" . symbol-overlay-mc-mark-all)))