Answer
Create a custom field called 'Customer Name' on the Business Opportunity entity — once that custom field exists, it becomes available in the 'More' dropdown of the search header's field picker. Aurena's search header field options are driven by the entity's actual attribute list (standard plus custom), so a computed value that IEE resolves through an internal function (rather than exposing as a real column) needs to be recreated as a custom field before it can be offered as a search header option in Aurena.