Shared lyrics with occasional short solos

[Peter B. Steiger]

I'm using Lilypond (1.6.6-1, installed from RPM binaries to Red Hat 7.3) 
primarily for 
my barbershop quartet music, and I'm having a little trouble finding the right 
way to 
show lyrics.

In a typical quartet piece, all four parts sing the same words *most of the 
time*, but 
there are spots where occasionally one part will hold a note out while the 
other three 
sing a short phrase.

A brief example is here:
<a href="http://www.steigerfamily.com/pbs/bbshop.jpg";> 
http://www.steigerfamily.com/pbs/bbshop.jpg </a>

I added the extra harmony part lyrics by hand to the image; everything else is 
as 
Lilypond rendered it.

I see from the example files how you can either have all four parts singing the 
same 
words (TrustAndObey.ly) or all four parts singing separate words (satb.ly), but 
I can't 
figure out how to manage this one.  What I have so far (without the extra 
lyrics) is in 
this source file:
<a href="http://www.steigerfamily.com/pbs/bbshop.ly";> 
http://www.steigerfamily.com/pbs/bbshop.ly </a>

I guess it's short enough to post here too, with irrelevant portions removed:

\include "english.ly"

\header {
    filename="bbshop.ly"
    title = "Barbershop Quartet Test"
    composer = "Lily the Lilypond Dweller"
    arranger = "The Lone Arranger"
}

global=\notes {
        \property Score.BarNumber \override  #'break-visibility = 
#end-of-line-invisible
        \key bf \major
        \time 4/4
}

tenor=\context Voice = tenor \notes \relative c' { r4  ef'8 ef  ef4 f     r  f  
g ef d1 }

lead=\context Voice = lead \notes \relative c' { f4  g8   a   g4  f     bf1 ~ 
bf1 }

baritone=\context Voice = baritone \notes { r4 c'8   a   bf4 a     r  af g  gf 
f1 }

bass=\context Voice = bass \notes \relative c { r4  c8   c   d4  c     r  d  df 
c  bf1 }

verseOne=\lyrics { My Li -- ly -- pond, it's fine----------. }

\score {
  <
    \addlyrics
    \context Staff = upper < 
      \context Voice=tenor <{\voiceOne \global} \tenor>
      \context Voice=lead <{\voiceTwo \global} \lead>
    >
    \context Lyrics \verseOne
    \context Staff = lower { 
      \clef "bass" <
        \context Voice=baritone <{\voiceOne \global} \baritone>
        \context Voice=bass <{ \voiceTwo \global} \bass>
      >
    }
  >
}

Can a smart person show me what I need to add to get the high tenor, baritone, 
and 
bass parts to show all the same lyrics as the lead all the way through to the 
end, but 
then add their own lyrics in the last measure (so that high tenor lyrics are 
above the 
top staff, baritone lyrics are above the bottom staff, and bass lyrics are 
below the 
bottom staff)?

Thanks!
pbs
--------
Peter B. Steiger
Cheyenne, WY