Paid when Paid - Contract Billing

We are in the process of testing the Paid when Paid Functionality across AR, Contract and Service Billing modules, and have been able to successfully complete the process through both AR (Manual Link), and Service Billing (Auto Link).  However, we are unable to link the Pay When Paid items against Billing Lines that are of type Lump Sum within Contract Billing. Documnetaion paints a broad picture, and states that it is avaiable across Contract and Service Billing, but I would assume this is only applicable for lines with an established x-ref to the Billing Lines.

Are we missing elements, or does this require a manual link similar to that of the AR Process?  Has anyone had any success in performing the PWP process within Contract Billing?