Pay Now button on Pro Forma Invoices

We use invoice payments via Stripe for SOP invoices but would like to extend this to pro forma invoices also. Currently it is not possible to add a Pay Now button to a pro forma invoice layout. The benefits of having this on pro forma invoices are that our customers can pay promptly so we can expedite the shipping process. These customers also tend to be B2C and so find it more convenient to pay by card. It would also allow us to check which pro forma invoices have been paid with greater ease, using the process invoice payments option rather than manually checking our bank to see which pro formas can be converted.