As you should be aware once you do PGI with movement type 601, the inventory gets knocked off from the system and you cannot have any traceability over this.
May be what you can do is create deliveries and just save based on which, you can certainly, create a proforma which can be filed with customs. Once the export formalities are over, you can do PGI and generate a commercial invoice.
The other option you could consider is to have one more storage location for port. As and when the goods are ready for despatch, you can transfer the goods with 311 movement type. But if this export scenario is from India, then you also need to take care of excise formalities.
G. Lakshmipathi