\version "2.19.30"

#(define (accidental2CommaFlat grob) 
   (grob-interpret-markup grob 
     (markup ;; Select Bravura font
            ; This doesn't seem to work (although I think it should)
            ; #:font-name "Bravura" 
             ;; set size of the glyph - 
             ; please experiment wtih appearance and problem of interference with staff lines
             #:fontsize 5
             ;; choose the glyph to be taken
             #:char #xE455))) 

\relative c'{
  % There has to be a key, otherwise nothing will be printed.
  \key g \major
  
  % Use Bravura, the SMuFL reference font (actually this should be done 
  % inside the stencil but I don't know how
  \override Staff.KeySignature.font-name = "Bravura"
  
  % Tell LilyPond to use something else than the default key signature
  \override Staff.KeySignature.stencil = #accidental2CommaFlat
  
  c
}