Answer
Yes — set up a Process Enrichment workflow that listens to the entity's own …_Default() projection action (for example PurchasePartSupplier_Default() on SupplierForPurchasePartHandling) with After timing, and have the workflow set the Discount value to null in that response. This works by changing what the API returns for default-value population in the UI, rather than intercepting or blocking the actual create call — the field stays genuinely mandatory, but nothing pre-fills it with 0, so users have to enter a real value themselves.