Do you expect this data to show up in the PDF or would this be just some meta information that is only visible to you? Based on your workaround, you probably prefer the former, correct?
I would suggest to put this information into the "salutation" or "payment terms" free text fields, or would this be a bad fit? You can also adjust the headings for those sections under "invoice types".
Or would you expect (or require) this information to show at the same place where it lists the invoice no, the invoice date etc...?