I am looking for the same thing. I have run across this answer so many times...

Here's what I (and others, I think) are looking for:

A way to allow the sale of SELECT items when the inventory falls below zero.

I keep a running US Dollar value for all the stock on hand. So I don't really want to add an inventory number to the items that will pre-ordered until I have them in hand.

Sold Out items say sold out. Pre-order items can still be sold with a stack value of <0

When going to admin->configuration->stock, there simply is one option missing "allow pre-orders on select items". This would then need to be a selectable field on the product (admin) page itself.

Can this be done?