Thank you Both for your suggestions,
I have tried Joram's approach by
1) adding the Bravura otf to my Lilypond installation by adding it in the following location :
Lilypond.app/Contents/Resources/share/fonts/otf/Bravura.otf
2)
I linked my .ly file to the snippets folder by including it with the following
\include "snippets/custom-music-fonts/smufl/definitions.ily"
3)
I try to use the glyph for the glyphname "noteheadClusterWholeBottom" with
%%%%%%%%%%%%
\version "2.18.2"
up = \new Staff = "up"
{
\smuflOn
s1 -\tweak #'Y-offset #2
_\markup \smuflglyph #"noteheadClusterWholeTop"
-\tweak #'Y-offset #1.1
_\markup \smuflglyph #"noteheadClusterWholeMiddle"
-\tweak #'Y-offset #0.5
_\markup \smuflglyph #"noteheadClusterWholeMiddle"
-\tweak #'Y-offset #-0.1
_\markup \smuflglyph #"noteheadClusterWholeMiddle"
-\tweak #'Y-offset #-0.5
_\markup \smuflglyph #"noteheadClusterWholeMiddle"
-\tweak #'Y-offset #-1.5
_\markup \smuflglyph #"noteheadClusterWholeBottom"
}
%%%%%%%%
but I get a bunch of "{" characters for each smuflGlyph instead of the cowell notation.