MRP management of Call Off Purchase Orders

Could MRP be developed to deal with Purchase Orders managed on a call off basis. Scenario: Sage User Relies on MRP for Purchase Order Recommendations. The Sage User has some products on purchase order that are managed on a &call off& basis. That is to say that they place a large order for the supplier to cover a number of months or a whole year and then &call off& (request delivery of) the goods, when they require them. Suggestion: Perhaps through use of a checkbox to identify the purchase order line as Call off along with intelligent use of the requested delivery dates MRP could make a &Call Off& Recommendation instead of either recommending a further BUY instruction or failing to see a shortfall due to the existing order.