I can't comment on "Revenue Canada" or "ServiceOntario", but "corporate America" definitely follows the adj+noun format.
In this case, the noun is "America", and "corporate" is an adjective that refers to a subset of that noun. It is used in the same manner as "rural America." "Corporate America" is different from "American companies" in that "American companies" refers to all companies in America, including mining, agri-business, manufacturing, and consumer sales, whereas "corporate America" is almost always limited to the stereotypic cubicle-dwelling office workers employed in the headquarters of large corporations.