Arranging lyrics between 2 staves

[David Sumbler]

The piece I am setting can be sung by a baritone or by a mezzo-soprano. 
 In the score, therefore, there are 2 vocal staves, one for each of the
alternative voices.  The staves contain identical music apart from an
octave diffence in pitch and different clefs.

It seems sensible to print the words only once, mid-way between the 2
staves.  But if the words are aligned to the notes in the upper stave
they appear too high.  If they are aligned to the music in the lower
stave the result is more acceptable, although the words are perhaps a
little lower than one would wish.

This is illustrated in the following example, where intentionally no
notes or clefs extend beyond the stave.

%%%%%%%%%%%%%%%%%%
\version "2.19.48"

%%%\language "english"

\new StaffGroup <<
  \new Staff {
    \new Voice = "upper" {
      \clef "bass" g2 g a1 } g2 g a1 \bar  "|." }
  \new Lyrics {
    \lyricsto "upper" { Words too high! } }
  \new Staff = "lower" {
    \clef "bass" a,2 a, g,1 \new Voice = "low" { a,2 a, g,1 } }
  \new Lyrics \with { alignAboveContext = "lower" } {
    \lyricsto "low" { Words too low? }
  }
>>
%%%%%%%%%%%%%%%%%%

Incidentally, I find similar problems with positioning dynamics between
the staves of a piano score.

The easy solution for me would be to use the second method, i.e.
aligning the words to the lower stave.  But there is a complication: 2
of the 6 songs in the piece are in Welsh, and I want to provide a
metrical English translation for singers who cannot or do not wish to
sing in Welsh.  My original idea was that I could attach the original
words to the upper stave, and the translation (in italics) to the
lower.  This would neatly have solved the spacing problem if only
Lilypond would arrange things symmetrically.

How can the exact vertical positioning of words (or dynamics) between 2
staves be controlled so that neither stave appears to be favoured?

David