Add "Billed" as an Event State

We have Draft, Tentative, Confirmed, Denied and Cancelled but what about the details that need to be taken care of after an event? How can we track what events we have billed? With the pricing and invoicing features, there is no way to run a report for what has been sent an "invoice" report. I believe most users would send a final bill after an event has taken place so to add an event state that could be trackable and searchable and the ability to track that data through reports, my team would love to see a "Billed" state added. The process would be that once the invoice was sent or our internal billing process was completed, we would move the event to the "billed" state. Searches can be set up by selecting event states, which would allow users to search for what has been billed.