Buyer Assignment Extension

Hello,

We want to be able to automatically assign approved purchase requisition lines to a buyer based on client specific logic.  For example, if the requisition line is project related, find the default project buyer in a DFF on the project accounting project definition. 

We will not have automated purchase order creation.  Buyers will use the standard autocreate form (or PBWC equivalent) to manually select the requisition lines to add to and create the standard purchase order. 

My question is, if we wanted our specific buyer assignment logic to fire, where would be the best place to put the logic?