the idea is to create a special FC Logging Agent that logs ACCESS(open) to documents and then to create Select List based on a SQL statement that uses the logging table to find a docid of the most recently opened document and from that ID, selects entry in the keyword table. Select list used in the stamp. Instead of SQL statement in the Select List you could use a database view created with the same logic (The view will show dwkeywords for DocID that corresponds to the most recently opened document from the Logging table)
If a single stamp/select list used for all users there is a statistical risk that another user opens another document between time 1st user opens a document and applies the stamp to it. If you want to prevent this - you can create separate Select lists for each of the stamping users, and issue individual stamps to iusers using that user's select list - - a lot of Administration effort, but does achieve your need.
Seems the same is possible in 7, but the logging table already there (named FCNAME_AUD).