Hi David,
my first idea was to change the grace note head stencil but, for some reason, I cannot change the ledger lin length:
parentGrace = {
\once\omit Flag
\once\omit Stem
\once\omit Accidental
\once\override NoteHead.stencil = #(lambda (grob)
(grob-interpret-markup grob
#{
\markup\concat\fontsize #-1 {
\musicglyph #"accidentals.leftparen"
\hspace #.3
\musicglyph #"accidentals.natural"
\hspace #.2
\musicglyph #"noteheads.s2"
\hspace #.2
\musicglyph #"accidentals.rightparen"
}
#}
))
}
{
\clef "altovarC"
%{
\once \override Glissando.minimum-length = #10
\once \override Glissando.springs-and-rods = #ly:spanner::set-spacing-rods
\once \override Glissando.bound-details.left.Y = #-3.25
\once \override Glissando.bound-details.right.Y = #-2.75
\once \override Glissando.bound-details.left.padding = #3.5
\once \override Glissando.bound-details.right.padding = #.8
%}
\pitchedTrill c2\startTrillSpan
%% shorter syntax:
%-\tweak minimum-length #8 % <= for a similar note to note distance
-\tweak minimum-length #10
-\tweak springs-and-rods #ly:spanner::set-spacing-rods
-\tweak bound-details.left.Y #-3.25
-\tweak bound-details.right.Y #-2.75
-\tweak bound-details.left.padding #3.5
-\tweak bound-details.right.padding #.8
\glissando ~ des
\parentGrace
\grace { d8 }
c8\stopTrillSpan r8
}