This is already possible. When on the payment window no need to make any selections or fill any fields just click "Next"

you already can.. we post like this all the time. don't select a method and it'll be fine.

Just hit "Next" without choosing a payment method; it should post. It highlights the drop-down for the method of payment, but for $0.00 invoices, it won't prevent posting if you do not select one.