Yes this can be done with Custom.pll It has a procedure call Event.In this procedure you can capture events like when-new-form-instance,when-new-item-instance etc So capture tht event on which you want to disable your DFF and use name_in('system.current_form/item') to know the name of your DFF and then using form built-in set_item_property disable that field. Same way we can do enable the DFF.