Hi Craig,
How about :
\version "2.19.36"
\paper {
left-margin = 25
}
\score {
\new ChoirStaff \with {
%\consists "Instrument_name_engraver" %% <= why ?
instrumentName = \markup {
\rotate #90 "SOLI"
}
shortInstrumentName = \markup {
\rotate #90 "SOLI" \hspace #3
}
}
<<
\new Staff = "alto" {
\set Staff.instrumentName = #"Alto"
\set Staff.shortInstrumentName = #"A."
\new Voice = "alto" {
\repeat unfold 20 { c''1 }
}
}
\new Staff = "tenor" {
\set Staff.instrumentName = #"Tenor"
\set Staff.shortInstrumentName = #"T."
\new Voice = "tenor" {
\clef bass
\repeat unfold 20 { c'1 }
}
}
\new Staff = "bass" {
\set Staff.instrumentName = #"Bass"
\set Staff.shortInstrumentName = #"B."
\new Voice = "bass" {
\clef bass
\repeat unfold 20 { c1 }
}
}
>>
\layout {
\context {
\Score
}
\context {
\ChoirStaff
\override InstrumentName.self-alignment-X = #LEFT
\remove System_start_delimiter_engraver
}
\context {
\Staff
\override InstrumentName.self-alignment-X = #RIGHT
}
}
}