(define (make-processor-operator op)
  (lambda args
    (lambda (processor)
      (modulo (for/fold ([res 0])
                        ([arg args])
               (match arg
                 [(? number?) (op res arg)]
                 [(? lens?) (op res (lens-view arg processor))]
                 [(? procedure?) (op res (arg processor))]))
              256))))

(define :+ (make-processor-operator +))
(define :- (make-processor-operator -))

(define (:= src dest)
  (lambda (processor)
    (lens-set src processor (dest processor))))

((:= A (:+ A 1)) (Processor 0))

I think your “lensify” idea turned out to be the better approach overall. For the sake of a : prefix (and maybe a little performance penalty due to more function calls) here and there, no macros necessary :)

again, I really don’t think I would have came up with this purely on my own, so thanks a lot.

I’m glad I could help :)