Bar numbering -- tweaking staves?

[Till Rettig]

Hello,

I started putting a ChoirStaff together with multimetre mesures. Because 
the score context calculates the bar numbering, it cannot gather anymore 
information on all staves, since they appear to have different measure 
numbers. One possibility is to add the bar_number_engraver to the staff 
context, but this invokes ugly numbers for every single staff. I still 
would prefer bar numbers on the top of the system, thus being valid only 
for the upper voice.
So how can i seperate one staff (or voice) to which i can apply then the 
bar_number_engraver, not affecting the other staves?

Greetings
Till

Example-file:

%%%Lily-file%%%
\version "2.8.4"
sopran = { \time 4/4 a b a c f g fis e d c g a d2 c e c }
alt = { \time 3/4 a4 b a c f g \time 4/4 e2 b  d f \time 3/4 a4 b a c g f }
\score {
\new ChoirStaff = Chor
\relative <<
\context Voice = sopran {\sopran \sopran}
\context Voice = alt {\alt \alt}
>>
}
\layout{
\context { \Score
    \remove "Timing_translator"
    %\consists "Bar_number_engraver"%doesn't show any effect
}
   \context {\Staff
    \consists "Timing_translator"
   \consists "Default_bar_line_engraver"
\consists "Bar_number_engraver"
}
}