Flag contacts as billable/not billable

We bill many of our agreements on a per-user basis. It would be nice to be able to flag contacts as billable (and have this be picked up by the contact counter in the invoicing module) or not billable. It would also be nice if this could extend to the contact card available on the left-hand side of each ticket, making it clear to our technicians that they needed to seek approval prior to performing work for non-billable contacts.