Answer
Check the underlying report package (RPI) source code for a call to Xml_Record_Writer_SYS.Add_Custom_Fields() — for example, Customer Order Confirmation's package (Customer_Order_Conf_RPI) calls Xml_Record_Writer_SYS.Add_Custom_Fields(xml_, 'CustomerOrderLine', ..., 'CUSTOMER_ORDER_CONF_REP', language_code_), which is what actually pulls Customer Order Line's custom fields (with Enabled On Reports checked) through into that report's XML/layout. If a report's package doesn't include this kind of call for the logical unit you need, that report genuinely doesn't support custom fields for that data — it's a real limitation of that specific Report Designer Layout, not something fixable through custom-field configuration alone; building an equivalent Crystal Report Layout instead is the usual alternative in that case.