For each member, there should be an area to keep notes.  I had been using a test forum thread to keep various notes on, now its archived (pita), and it was subject to public viewing.  Maybe a tab next to the 'archives' for a single thread that can be seen by the logged in member only, NOT the rest of the ARF.

If need be, sell it as an add on to a membership $2-3/year (whatever) to cover bandwidth...