TRANSPOSITION = ##f

transposeGlobal =
#(define-void-function (from to) (ly:pitch? ly:pitch?)
   (set! TRANSPOSITION (cons from to)))

transposable =
#(define-music-function (music) (ly:music?)
  (if TRANSPOSITION
    #{
      \transpose #(car TRANSPOSITION) #(cdr TRANSPOSITION) #music
    #}
    music))