The key is you want to show the inventory on your books. If this was not the case then you could treat it as consignment. If you don't have the consignment module you could have used an ASN to show the goods are on the way. But either one does not physically make them yours. I would create a new location and as you say, set it up as available but not allocatable. I would also set up a new status code to restrict which transactions can use this location to stop it being issued manually to say a WO. Finally I would probably put the PO number into the ref or lot field to keep each receipt seperate. Then once the good arrive, transfer it to a regular location and lose the lot/ref. If there is a balance (-ve or +ve) for that inventory, you can reopen the PO line and make the adjustments.