\version "2.14.2"

#(define (stencil-plus-bracket grob)
 (let* ((beam (ly:beam::print grob))
        (beam-count (length (cdr (ly:grob-property (ly:grob-parent grob X) 'beaming))))
        (beam-positions (ly:grob-property grob 'positions))
        (beam-dir (ly:grob-property grob 'direction))
        (gap (* 0.5 (ly:grob-property grob 'gap)))
        (single-beam-thickness (ly:grob-property grob 'beam-thickness))
        (whole-beam-thickness
          (+ (* beam-count single-beam-thickness)
             (* (- beam-count 1) gap)))
        (bracket (ly:bracket Y
                   (cons (* -0.5 whole-beam-thickness)
                         (* 0.5 whole-beam-thickness))
                   0.05 0.4)))

   (ly:stencil-add
     beam
     (ly:stencil-in-color
       (ly:stencil-translate
         bracket
         (cons -1
               (+ (car beam-positions)
                  (* beam-dir -0.5 whole-beam-thickness)
                  (* beam-dir 0.5 single-beam-thickness))))
       1 0 0))))

\relative c' {
  %\stemUp
  \override Beam #'stencil = #stencil-plus-bracket
  f'32 [f f f]
  f16 [f f f]
  f64 [f f f]
}