Graphic background behind systems

[Paul Booker]

%{
I am trying to make an image using a number of single-system scores wrapped
in \markup blocks, 
and I would like to place the samplePath graphic behind each barline (i.e.
drawn first in svg output),
but remain independent so I can position them in \hspace.
Ultimately the path graphics will be longer, spanning more systems, 
hence the proportional spacing to keep them aligned.
I haven't been able to get layers working, nor collison detection turned off.
Is that because they are not staff related objects?

The graphics will also be "not black" one day, but would it also be possible
to have 
white-out on the barline and staff to help it stand out?
Many thanks
Paul
%}

\version "2.19.36"
samplePath =
  #'((moveto 0 0)
     (rcurveto 0 1 1 1 1 0)
     (rlineto 0 -5)
     (rcurveto 0 -1 -1 -1 -1 0)
     (closepath))
\markup {
        \hspace #33
        \override #'(filled . #t) \path #0.1 #samplePath
        \hspace #18.2
        \override #'(filled . #t) \path #0.1 #samplePath
        \hspace #18.2
        \override #'(filled . #t) \path #0.1 #samplePath
}
\markup {
        \score {
                \new StaffGroup
                \new Staff \with { 
                        \omit Staff.TimeSignature
                } 
                \relative c {
                        \set Score.proportionalNotationDuration = 
#(ly:make-moment 1/8)
                        \override Score.SpacingSpanner.strict-note-spacing = ##t
                        c'1 | c | c | c |  
                }
                \layout { }
        }
}