Require PO on Invoice by Supplier

Summary:

We are looking for a field at the Supplier or Supplier Site level which we can flip to indicate whether a Supplier is considered a PO Invoice Supplier, Non-PO Invoice Supplier or Either/Both.

We would then like Create Invoice screen to require the "Identifying PO" field if the Supplier selected has this "PO Invoice Supplier" flag set.

We've tried using Supplier - Site - Invoicing (tab) fields of Invoice Match Option = Order and Match Approval Level = 2-Way but these fields appear to be defaults rather than a hard stop and they still allow Non-PO invoices to be entered against those Suppliers.