Answer
The Project Invoices Logical Unit is specifically listed among the framework's Excluded Logical Units, which blocks custom-field extension on it regardless of how closely its underlying view relates to the extendable Invoice LU's own table. The specific R&D rationale for excluding this particular LU from custom-field support isn't publicly documented, so if there's a genuine business need for a custom field there, the practical path is to raise a support case asking IFS R&D directly for clarification or reconsideration, rather than assuming it can be worked around through configuration alone.