question about cross-staff stems

[james]

Is it possible to get cross-staff stems to ignore a voice? I'd like to have the 
lowest voice here completely ignored for the cross-staff stems, alternatively, 
have the cross staff stems connected to the upper voice. As it is, the only way 
I can the stems to behave the way I want is to flip the direction of the stems 
on the lowest, which, given the ties and the spacing is not an ideal situation.

\include "deutsch.ly"
\version "2.19.65"

\score {
        \new PianoStaff <<
                \new Staff = "RH" \relative c'' {
                        \voiceOne <c c'>4~ q8 q~ q4~ q8 q
                }
                \new Staff = "LH" \relative c' {
                        \clef bass
                        <<
                                \new Voice = "crossing" { \autoBeamOff 
\crossStaff \repeat unfold 2 { \change Staff = "RH" \voiceTwo s8 <c e>8 <c f> 
<c a'> } }
                                \new Voice = "notCrossing" { \autoBeamOff 
\change Staff = "LH" \voiceTwo \repeat unfold 2 { c,8[ g'] g[ g] } }
                                \new Voice = "ties" { \voiceFour \repeat unfold 
2 { \stemDown c,4~ c8 r } }
                        >> \oneVoice
                }
        >>
        \layout { \context { \PianoStaff \consists #Span_stem_engraver } }
}

Thanks,

James