flats and double flats on a RhythmicStaff

[David Raleigh Arnold]

How do I get a flat on a rhythmic staff?  This is for harmonica,
indicating bent notes.  It makes a lot of sense as a way to
indicate it, and some indication is necessary.  (There are two g,'s,
one can be bent and the other cannot.)

\include "english.ly"
\include "paper20.ly"
\score { 
\context RhythmicStaff = one 
\notes { \clef "G_8"
\property Staff.TimeSignature \override #'style = #'() 
\property Voice.Stem \override #'thickness = #1.0
\stemDown
\property Staff.instrument = "\\Large{11}"
 \time 4/4
 c,4^"1"^" "  c,^"1"^" "  c,4^"1"^" "  c,^"1"^" "  
d,^"\\large\\textcircled{\\small{1}}"^" " 
g,4^"3"^" "  gff!,2^"\\large\\textcircled{\\small{2}}"^" "  
}  
\midi{}
\paper{linewidth=7.0\in indent=0.15\in}
}