I would likely request it as an IPA. First Choice: as part of req approval, if the vendor on the line is in the list of vendors, change buyer code. Second Choice: When record is added to POINTERFAC (PO23), if vendor in the list of vendors, change to buyer code. You'll likely have to maintain the list in Landmark as a task or as a custom table.
We have also changed the issue method of some vendors to email a specific buyer. This allows the batch and release to go through the PO120 runs send it to the correct buyer.