I've just now thought of this: the Highlights in the Annotations browser have a timestamp and possibly a coded "position". I'm sure this would mean a lot more work in your plugin and would be rarely used by most people, but if it were able to pull the last highlight position (actually the nearest one towards the end of the book), this would make a very interesting reading progress pointer for me. :P
(also, not sure if it would be feasible to somehow translate that highlight position into some #percentage/progress column). Just a thought...
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