\version "2.18.2"

#(define (bracket-stencils grob)
  (let ((lp (grob-interpret-markup grob (markup #:translate (cons -0.3  -0.35) "[")))
        (rp (grob-interpret-markup grob (markup #:translate (cons -0    -0.35) "]"))))
    (list lp rp)))

{
  c'2-\parenthesize-!
  \once \override ParenthesesItem.stencils = #bracket-stencils
  c'2-\parenthesize-!
  c''2-\parenthesize-!
  \once \override ParenthesesItem.stencils = #bracket-stencils
  c''2-\parenthesize-!
}