Oracle subquery column aliasing syntax?
Consider using the WITH syntax that has the subquery above the outer SELECT and has the better maintainability you were looking for: 

WITH subquery (a, b) AS 
(
    SELECT x, y FROM whatever
    UNION
    SELECT z, w FROM something
) 
SELECT a, b FROM subquery;


... runs on Oracle 11.2 according to: http://sqlfiddle.com/#!4/d0b42/2/0