Okay, I did a Fire & Forget on this module. Apologies to the community!!
I cannot edit my original post, so here is a link to the addon:
Product Special Order.
Just added v1.1, which has tidier code, better instructions and has been confirmed to work in 1.3.9+.
Reading the comments in this thread, I realize that I never bothered to include in my ReadMe how to actually USE the modifications, so I have to apologize (again) for that.
In a nutshell, you need to set the Product Quantity to "-100" to trigger the Special Order flag. I didn't state this in my instructions! If you set the threshold to a lower number (-199) instead of -99, then set products to -200 in order to trigger it, etc.
I will see what I can do about modifying the email content for rjspomer. For RobertG, I don't follow you... please elaborate.