Display of Non-Billable or Zero Dollar Transactions on a Customer Invoice

Does anyone have any suggestions for how to display non-billable or zero dollar billed transactions on a customer invoices.  In either case, there appears to be no link from these transactions to a draft invoice.  I tried setting the Invoice Net Zero Lines option (in Implementation Options as of 12.1.3), but it doesn't seem to provide any ability to do this either.  Is there anything I'm missing in the back end that would provide linkages to a draft invoice (that I could reference in a custom invoice print) or is the only option to create a separate custom report to show