\version "2.14.1"
\paper { tagline = ##f }
#(define (label text)
  (lambda (grob)
    ;; adapted from LSR snippet, "Center text below hairpin dynamics"
    (ly:stencil-aligned-to
      (ly:stencil-combine-at-edge
        (ly:stencil-aligned-to (ly:horizontal-bracket::print grob) X CENTER)
        Y (ly:grob-property grob 'direction)
        (ly:stencil-aligned-to (grob-interpret-markup grob (markup text)) X CENTER)
      0.2)
     X LEFT)))

\layout {
  \context {
    \Voice
    \consists "Horizontal_bracket_engraver"
  }
}

\relative c'' {
  \override  HorizontalBracket #'direction = #UP
  c4-\tweak #'stencil #(label "A")\startGroup
    -\tweak #'stencil #(label "a")\startGroup
  d4\stopGroup
  %% following tweak seems to have no effect
  e4-\tweak #'stencil #(label "b")\startGroup
  d4\stopGroup\stopGroup
}