Can you assign a AR fulfillment set using a defaulting rule using Customer and Fulfilllment set.

We have an item and a Rebate item applied to an order using the Promotional Goods modifier. The Rebate item is the "Get Item" and has an item of "shippable" so it flows through to ship confirm with the shippable item. If I have a partially shipped line(split line), the quanitity of the REBATE item is not automatically adjusted to match the partially shipped line. We are thinking of using an AR fulfillment set to ensure the item and REBATE item invoice together and the user can adjust the REBATE item, when the item is partially shipped before the two