There's no AR equivalent, you're reading it right. AR Preferences only offers Require Payment Reference on Entry, and that applies to Payments and Applications (AR302000) and Cash Sales (AR304000), not to invoices.
The only Require Customer Order Nbr. setting Acumatica ships sits on Order Types (SO201000), so it covers sales orders and never reaches an invoice you raise in AR or from project billing.
You can enforce it with a small customization, no code needed. In the Customization Project Editor, add a condition on the Conditions page (Type Equals Invoice, so credit memos don't get blocked), then on the Fields page set that condition as the Required property of the Customer Order Nbr. field (ARInvoice.InvoiceNbr). Do it at screen level. At DAC level it applies anywhere ARInvoice is used, including invoices generated by project billing and anything arriving through import scenarios or the API, and those will start failing to save.
If you bill through pro formas, PM307000 has its own Customer Order Nbr. box, so capture it there before you release.
On amending after release, there's no stock way. Once an invoice is released the only editable fields are the cash discount date and due date while it's open, so a credit memo is the only route today without further customization.