(define-syntax postfixed
  (syntax-rules ()
    [(_ (operands ... operator)) 
     (operator (postfixed operands) ...)]
    [(_ atom) atom]))

;; all of these return 5
(postfixed 5)
(postfixed (2 3 +))
(postfixed (2 (1 2 +) +))
(postfixed ((1 1 +) (1 2 +) +))