Free shipping a separate column instead of just 0 weight

[Pr0ject]

I was wondering if there was anyone out there that knows how the free shipping for 0 lb product works and how I could change it to check for an extra field value instead of a 0 lb weight issue.
The reason is because feeders that require/want me to send out the correct weight of the product and have a different field to specify free shipping.
if someone can let me know the file where the free shipping for 0 lb product is at I could add an extra field for it to check and I could just flag that field as positive or whatever.

[Pr0ject]

In addition, the following bug occurs when you combine free and non free shipping items into the same cart.
Free Shipping Items Mixed with Standard Shipping items