Hi Andrew,
How about:
\version "2.20.0"
\layout {
\context {
\Score
\name Score
\accepts QuartetGroup
}
\context {
\StaffGroup
\name QuartetGroup
\accepts TrioGroup
}
\context {
\StaffGroup
\override SpanBar.transparent = ##f
\name TrioGroup
\remove System_start_delimiter_engraver
}
}
\score {
\new QuartetGroup <<
\new TrioGroup <<
\new Staff { \repeat unfold 10 s1 }
\new Staff { \repeat unfold 10 s1 }
\new Staff \with { instrumentName = "Viola" } {
\repeat unfold 10 s1 }
>>
\new Staff \with { instrumentName = "Cello" }
{
\repeat unfold 5 s1
\once\override QuartetGroup.SpanBar.transparent = ##t
\repeat unfold 5 s1
}
>>
}
HTH, cheers,
Pierre