The closest I get is via the Web Services - Pag 526 - Posted Sales Invoice Lines that contains a field called /Document_No_link/, which contains the /Order_No/ after expanding. But that Order_No is actually the Order_No that is contained in Invoice Header. But that field is only filled out if you invoice from the sales order directly, not when you manually create an invoice and add lines linked to a sales order. In other words, the /Order_No/ field in the header is not really that useful.
Any pointers would really be appreciated!