Look into assigning a W/D/R number instead (Waiver, Deviation, Rejection).  Assigning this value tags the part or lot with a unique identifier, then you could begin to restrict the movement on this basis.  You would need an event at some point in the Reservation or Picking process to detect whether the W/D/R value was connected to the correct customer. 

I agree, Availability Code won’t do what you want.  W/D/R might work.  You might also have to use a custom field and possibly a custom table to provide the allowed connections of the customer to product, but then still would need a check of the order to make sure it is valid before invoicing.