Different notehead types sharing a stem in double-stops

[Michael Culbertson]

Hello,

  For string instruments, double-stops that involve all four chords are usually
written with quarter-note heads for the bottom two notes, even if the rest of
the chord is a half note.  This is because the bottom two chords are struck only
momentarily, like grace notes.  Consider, for example:

\paper { ragged-right = ##t }
\new Score \relative c' {
  \clef treble \key g \minor \time 3/4 \cadenzaOn
  r4\< \times 2/3 { d'8[ e\tenuto( f\tenuto)] } \bar "|"
                <g,, d' bes' g'>2\f g''4 g \bar "|"
  \cadenzaOff
}

Here all four heads should touch the same upward stem, and the lower G and D
should have filled-in heads, while the B-flat and upper G should have open
heads.  But, I can't figure out how to entice Lilypond to do such a thing.  I
tried separate voices, but Lilypond either split the stems (reasonable if they
actually were different parts) or complained of note collisions (when I tried
forcing the stem directions).  In any case, the crescendo wouldn't end on the
forte with the anonymous separate voices hack.  I figure there's a way to
override the note heads, but I can't make out the necessary scheme.  Any ideas?

Thanks,
Michael Culbertson