For BigCommerce you can mark a products as a core product and we can use an option flag to set them all to 999 stock. BigCommerce does not have an option to allow oversell or zero stock products Even for Preorder!

In testing an annoying “fault” with the pre order functionality concerns stock levels. It does not work for products unless there is a stock level in there. This does not make much sense but pre-order is core functionality it is not controlled by the theme. 

If a product has been changed to core product, the stock will not be updated to match (as of at least 01/09/2022). The following SQL snippet adds all incorrectly 'stocked' core products into the queue to have their stock updated.

INSERT INTO transfer_queue 
    1000 - B.inventory_level
FROM rproduct AS ir
    inner join BProduct_Variant B on ir.SKU = B.SKU
WHERE IR.CoreProduct = 1
    and B.inventory_level <500