Customized rejection option in APINV

Has anyone here successfully modified Payables Approvals (APINV) to offer more than one rejection option?

I manage a relatively large and complex approvals system for a multi-org setup dealing with over a million invoices for approvals per year. One of the major complaints is that it’s taking the bookkeepers a long time to sift through rejected invoices to see the reason for rejection (we have a modification requiring entry in the notes section of the response if an invoice is rejected) and then respond accordingly. The reasons for rejection can be varied, but we want to catch the ones that the approver thinks is wrongly accounted.