Bill Type Equivalent for Sub Invoices

We use Bill Types to identify whether or not a bill has already been paid or if it still needs to be paid. This helps prevent duplicate payments when syncing the bills to our AP processing system. It would be helpful to have a Bill Type field in Sub Invoices so that we can quickly see if it has been paid or not. Internal Comments won't work because they don't show up as a column option in the Procurement module like the Bill Types do.