Hi Lee,
for release 1602 it is planned to support service order items not relevant for invoicing. This means that you can then create a dummy customer for those internal service orders, and then costs are collected on those service order items, but no revenue is generated and they should not show up in the customer invoice request list.
Of course release 1602 is not yet officially released, so there is no comittment, that 1602 will include this feature. Please wait for the official release of 1602 and then check, if this fulfills your requirement.
Best regards,
Stefan