Using code from Custom fields in partial_invoice_table.php, I added columns with custom or looked-up values to either quote and invoice which worked well but when I went to the client form, it crashed complaining about the partial_invoice_table.php or partial_quote_table.php changes I did.
I had to revert all changes to get the client page to work again.
Luckily, the same edits to the dashboard are supported!
I guess this is because the invoice, quote and payments views are ‘linked’ to the client view but do not succeed to fix this dependency.