Per-voice instrument name

[Dan Eble]

I'd like to be able to define an instrument name per voice and see the names 
rendered in a column.  I have a kludge for this that led me to discover issue 
4213.  Rather than going into any detail about that, I'll simply suggest some 
Lilypondish input (which does not currently work) and request advice on the 
best approach.  Any ideas?

\version "2.19.15"

aNotes = {
  % I don't care much about the details of the syntax; the key idea is
  % to keep the part name with the music for the part.
  \set instrumentName = "S"
  e'
}

bNotes = {
  \set instrumentName = "A"
  c'
}

<<
  \new Staff \with {
    \remove Instrument_name_engraver
  } <<

    \new Voice \with {
      \consists Instrument_name_engraver
      \voiceOne
    } { \aNotes }

    \new Voice \with {
      \consists Instrument_name_engraver
      \voiceTwo
    } { \bNotes }
    
  >>
>>
— 
Dan