Currently, the only callouts available for credit memos are "Credit Memo Created", "Credit Memo Posted", and "Email Credit Memo". It would be very useful for us to have a callout when a credit memo is applied. We need to take an action when an invoice's balance is brought to $0. For payments, there's a callout for "Payment Processed", which works for that case, but there's no way for us to know when a credit memo is applied to an invoice.
