Vertical alignment bug - Instrument_name_engraver

[Daniel Johnson]

Instrument_name_engraver has a problem calculating the correct Y-extent 
to which it applies, when it is applied to a staff-group rather than a 
single staff, and the bottom staff has lyrics attached to it.  It 
calculates the Y-extent as being from the top staff to the bottom of the 
lyrics on the bottom staff.  Consider the following snippet:

%%% BEGIN LILYPOND CODE %%%
\version "2.6.3" %Also tested with 2.7.12

\paper { raggedright = ##t }

text = \lyricmode { La La La }

\score {
   <<
       \context ChoirStaff <<
           \context Staff = highVoices {
               \set Staff.instrument = "Soprano"
               \set Staff.instr = "S"
               \context Voice = sop {c'1 \break c'1 c'1}
           }
           \context Lyrics = sop \lyricsto sop \text
           \context InnerChoirStaff = lowVoices \with {\consists 
"Instrument_name_engraver"} {
               \set InnerChoirStaff.instrument = "Bass"
               \set InnerChoirStaff.instr = "B"
               <<
                   \context Staff = baritone {
                       \clef bass
                       \context Voice = bari { g1 a1 g1 }
                   }
                   \context Lyrics = bari \lyricsto bari \text
                   \context Staff = bass {
                       \clef bass
                       \context Voice = bass { c1 f1 c1 }
                   }
                   \context Lyrics = bass \lyricsto bass \text
               >>
           }
       >>
   >>
}
%%% END LILYPOND CODE %%%

The output of this snippet (using 2.6.3) is here:
http://www.theologian.org/ly/inner-choir-staff.png
As you can see, the InnerChoirStaff instrument name (and its 
abbreviation) are shifted down to compensate for the presence of lyrics 
below the bottom staff.  Remove the bass lyrics, and everything looks 
normal.

I tried this using 2.7.12, and it repeated the incorrect output, except 
that it also mysteriously omitted the lyrics to both low parts on the 
first line (it picked them up on the second).  Lyrics to the soprano 
part remained correct.

As an aside, it seems to me that the use of an InnerChoirStaff is to 
indicate a subdivision of voices (primarily choral, but could be used to 
indicate 1st vs. 2nd violins, etc).  The standard in choral music is to 
include a small simple square-bracket outside the choir bracket.  I have 
placed an example here:
http://www.theologian.org/ly/inner-choir-staff-delim.png
How much work and how many euros would it take to implement this?  My 
ship may finally be coming in and I just might have a Lilypond category 
in my budget.

--Daniel