Let's say you have orders for 20TH and 10TH arrives with the first hardware delivery.
Instead of an even split, could you fill the orders proportional to the hashing power that was purchased? In other words, I purchase 1TH (5% of this example). That means I would get 500GH up and running with the first shipment. That seems more equitable than an even split in regards to the people who purchased large orders.
Should be an interesting week!

That was my first suggestion actually, "I do have a few ideas however, split the hashing equally. For instance.. let's say we only get .. 5x 1.2 TH miners, so for simple that that is 6 TH/s. We end up selling 12 TH/s by the time the equipment arrives. We could split everyone in 1/2 until the next order comes and cut off sales until then."
You definitely worded it better than myself; though I am on a phone too so please do excuse bad grammar and poor sentence structure. It's very time consuming to reply on a phone!

But that was where I was getting at with the quoted sentence was that the orders would be proportionally filled until new miners come in to fill up the original order and then new pre-orders could then take place. If we end up getting less than we sold out, new orders would either be postponed or they would be on 'backorder' or 'pre-order' for upcoming shipments and fulfilled the same way.