\version "2.18.2"

% Probably better as a vector lookup
#(define letter-names '("C" "D" "E" "F" "G" "A" "B"))

#(define alteration-symbols
   `(
      (-1/2 . ,(make-flat-markup))
      (0 . ,(make-null-markup))
      (1/2 . ,(make-sharp-markup))
      ))

#(define (format-key p)
   (let* (;(p (ly:music-property ne 'pitch))
           (nn (ly:pitch-notename p))
           (alt (ly:pitch-alteration p))
           (letter (list-ref letter-names nn))
           (alteration (ly:assoc-get alt alteration-symbols)))
     #{
       \markup \concat {
         #letter #alteration
       }
     #}))

#(define-markup-command (firstNoteAndKey layout props mus)
   (ly:music?)
   (let* ((keey (extract-named-music mus 'KeyChangeEvent))
          (tonic (ly:music-property (car keey) 'tonic))
          (notes (extract-named-music mus 'NoteEvent))
          (first-ne (car notes))
          (first-pitch (ly:music-property first-ne 'pitch)))
     (interpret-markup layout props
       (make-concat-markup
        (list
         (format-key first-pitch)
         "/"
         (format-key tonic))))))

amazingGrace = \relative c' {
  \key g \major
  \time 3/4
  \partial 4 d4
  g2 b8 g
  b2 a4
  g2 e4
  d2
}

\markup \bold \firstNoteAndKey #amazingGrace

{
  \amazingGrace
}

{
  \override Score.RehearsalMark.break-align-symbols =
  #'(clef staff-bar key-signature)
  % CENTER doesn't center over clef?!
  \once \override Score.RehearsalMark.self-alignment-X = #0.5
  \mark \markup \firstNoteAndKey #amazingGrace
  \amazingGrace
}

\paper {
  indent = 0
}