When a selection is made, could all other matching instances of this text also be highlighted?
(Image shamelessly stolen from the interwebs)
As a simple implementation (& to avoid adding "yet another" feature) perhaps the existing find mechanism could be used?
Doubleclick to select a word or maybe some combination of alt while selecting
Find panel is automatically engaged, below, with the selection populated. Thus all instances of this are now highlighted (Note that the find panel is not focused. You are still editing the document)
If editing of the document resumes and the find panel has not been interacted with then it automatically hides once more
A tremendous thanks for your work so far on such an amazing editor. The number of feature requests you get per day is pretty scary :S