making sure i'm not missing anything - there's not a way natively (and without scripting) to input data on an item fulfillment and have it source back to the sales order line after saving, right? in this case we need to add a BOL number to an IF and then ultimately have it show up on an Invoice PDF

if workflow allowed you set sublist level field then you could have used workflow to do this. Only option is I guess scripting.

You can def script this