I was scrolling through old Agile posts and found your question today. Did you ever receive an answer? (I know it was almost two years ago.)
An any case, the only constraints you can put on a board's columns are the number of Issues, either with or without sub-tasks included. However..the boards are run on filters, so you may be able to create a filter that uses the "Updated" field to limit the issues that are pulled into the board.
Hope this helps, if so could you mark the answer "accepted" so others can find it and it no longer shows unsolved?