I'm not 100% sure, but I think you should be able to do this using a negative value for the product price (ie. -1) and then using a macro to transform this to the text you want.
An alternative would be to extend the SKU object and add a POA field to it. You would then check this field before displaying the price. This method has the added advantage in that a real price can be stored with the product, so the store admin can assign it to an order easily.