This per-user categorization tree could be implemented using the plans for a new category system in this thread [1]. It is intended that each package or package instance could build its own tree based on the main category tree. There is no reason each user could not have one too.
[1] https://openacs.org/bboard/q-and-a-fetch-msg.tcl?msg_id=0003xq&topic_id=12&topic=OpenACS%204%2e0%20Design
