Hi @chimpchimp ,
Thank you very much for your post.
Enable Back Orders option only is available in sellable BOM products.
If the BOM product is not sellable the back orders option is not available but the stock can change to negative values if the BOM Stock Control option is disabled (calculated stock).
If you want to get BOM products with negative values using the BOM Stock Control, just enable the "Allow Back Orders" option in the parent product.
I hope this helps and let us know if you have any further question.