You could call a macro when users select from the list (Trigger). You would also need to have the settings to allow for writing to the disk. In the macro append to a file the selected value. You can get the selected value with GetFieldSelections(). If users can select multiple items you might want to add each selected field to the log using GetSelectedCount() and subfield().
Can't really think of a good method to keep the log from growing and growing. You could use e.g. logparser (a free Microsoft tool) called by the windows scheduler to take care of this. Add the log date into the log file for this to work.
