Can't add refund against completed pledge

One situation our users hit is that sometimes they have to refund part of a completed pledge. They do this by negative contribution - so the payment is recorded on the right date and shows up with other contributions (cancelling it wouldn't show that it has been paid & then refunded)

However, the UI doesn't allow a payment to be recorded against a completed pledge - this is arguably not just an issue for refunds - ie. if someone pledges 4 payments but makes 5 there is no ability to edit the pledge for this once the status has hit 'completed'

Joe, I've put this to you in the first place in case you have any comment - I will solicit comments for you & Dave but since it looks like I can't get this into 4.6 I probably won't take it further in a hurry.

In the real world we also find that payments come in for cancelled pledges - which we can't currently record