Hi ken
If all the products are in stock, the visitor that has not provided their address upfront could start adding products to the cart thinking that they are available and, when entering their address later, all of them would be removed automatically and they would be noticed that are "out of stock". Something that, in our opinion, is not good for conversions and can lead to higher abandons and we discarded this option from the beginning.
So, if you think this is better for your specific case and need it to work in that way anyway, you could ask a developer to assist you with this customization.
Best Regards,
Salva.