I'm surprised how little additional information, like Project Custom Fields, can be added to invoices. We have multiple billing codes we need to include on invoices and the only choice I see is to cram them all into the project name. Not ideal or pretty result.
The first version on invoicing focused on getting the workflow working end to end with some key data. But now that that is working letting more data follow through to invoices would help for many scenarios.
For how you are using the data, would it go to the whole sheet, line item group headings, line items, or attached as supplemental information?