replaceRest = #(define-music-function (parser location stil) (ly:stencil?) 
  #{
    \once \override Staff.Rest #'stencil = $stil 
    \once \override Staff.Rest #'X-extent = #(ly:stencil-extent $stil X) 
    \once \override Staff.Rest #'Y-extent = #(ly:stencil-extent $stil Y) 
  #})

ReplaceRest = #(define-music-function (parser location stil) (ly:stencil?) 
  ((ly:music-function-extract replaceRest) 
    parser location (box-stencil stil 0 0))
  )

openSquigStencil = 
#(ly:make-stencil
  `(path 0.3
    `(rmoveto 0 -1
      rcurveto 1 -4 4 4 8 -4
      ))
  (cons 0 8)
  (cons -5 -1)
  )

\relative c' {
  \clef "alto"
  \cadenzaOn
  c1 g1
  c1 r1
  c1 \replaceRest #openSquigStencil r1
  c1 g1
}