Barlines in a StaffGroup, using polymetric notation.

[Jethro Van Thuyne]

When using polymetric notation, is there a possibility to 
apply \bar ":" only to one staff, without seeing it appear 
on the other staves?

I'm scoring a piece for viola, contrabass and a piano 
playing in 3/4, with a soprano singing in 9/8, using:

 \set Staff.timeSignatureFraction = #'(9 . 8)
 \compressMusic #'(2 . 3) { ... }

For the sake of clear reading, I want to add \bar ":" to 
some places in the soprano part (e.g. every three eighth 
notes), but the dotted barlines keep appearing in all of the 
other staves in the StaffGroup too, which is not really what 
I want.

Is there some kind of tweak for this?

Kind regards,

Jethro Van Thuyne,
Antwerp, Belgium.