Answer
A Quick Report built with the data you need is automatically exposed as its own REST/OData endpoint without any Projection Configuration work required — this covers exactly this kind of "just get me these combined columns" integration need. One important operational detail: a Quick Report's generated REST endpoint name includes its randomly-assigned ID number, so to keep the same endpoint name stable across environments (TEST, PROD, etc.), transfer the Quick Report between environments using an Application Configuration Package rather than manually recreating it in each environment, which would give it a different ID and therefore a different endpoint name.