Multi-measure rests and counter in piano staff

[Helge Kruse]

When playing in an orchestra it's important to count rests. To make this 
tasks easier you can write numbers into rest measures.

Especially for a piano staff it gives a calm picture when the empty 
space between the staves is used. The attached picture shows this 
starting at Allegretto. Further it would be good to have the number of 
multi-measure rests between the staves.

How can I write _one_ multi measure rest number in piano staff between 
the staff lines?
How can I add a counter starting at a measure n and counting to measure 
n+x? This should not be the current bar number but an arbitrary counter. 
If this is not supported, how can I place the counter numbers to the 
middle of the piano staff?

Regards
Helge

\version "2.16.0"

up = \relative c' {
  R1*20 \mark "E" R1*26 | R1*3
  \time 6/4
  \tempo Allegretto
  a'2.\ppp r | r a | r a
}

down = \relative c {
  \clef bass
  R1*20  R1*26 | R1*3^\markup{rall.}
  \time 6/4
  a'2. r | r a | r a
}

\score {
  \new PianoStaff  {
    <<
      \new Staff { \compressFullBarRests \up }
      \new Staff { \compressFullBarRests \down }
    >>
  }
}

IMG.pdf
Description: Adobe PDF document