Measure_grouping_engraver on score-level

[Stefan Thomas]

Dear lilypond-users,
I tried to get the Measure_grouping_engraver on the score level, but it looks, in my opinion, not quite convincing.
In the below quoted snippet, the measure-groups should always bee on the top of the score, above all staves,like the metronome marks, for example. But the measure groupe appear above sometimes above the bassclarinet, sometimes above the horns, which doesn't look really nice.
How can I get the desired output. 
\version "2.13.0"

dreizwozwoachtel = { 
        #(set-time-signature 7 8 '(3 2 2))
        #(override-auto-beam-setting '(end * * 7 8) 3 8 'Score )
        #(override-auto-beam-setting '(end * * 7 8) 5 8 'Score )
        #(override-auto-beam-setting '(end * * 7 8) 7 8 'Score )}

neunachtel = { 
   #(set-time-signature 9 8 '(3 3 3))
  #(override-auto-beam-setting '(end * * 9 8) 3 8 'Score)
        #(override-auto-beam-setting '(end * * 9 8) 6 8 'Score)
#(override-auto-beam-setting '(end * * 9 8) 9 8 'Score)}

\layout
{ 
         \context { \RemoveEmptyStaffContext }
         \context { \Score  \override VerticalAxisGroup #'remove-first = ##t }
         \context { \Score \consists  "Time_signature_engraver" }
       \context { \Score \override TimeSignature #'style = #'() }
       \context { \Score  \override TimeSignature #'break-align-symbol = #'staff-bar }
        \context { \Score \override TimeSignature #'X-offset: = #ly:self-alignment-interface::x-aligned-on-self }
       \context { \Score \override TimeSignature #'self-alignment-X = #CENTER }
       \context { \Score  \override TimeSignature #'break-visibility = #end-of-line-invisible }
       \context { \Score \override TimeSignature #'extra-spacing-width = #'(+inf.0 . -inf.0) }
       \context { \Score \override TimeSignature #'font-size = #3 }
        \context { \Score \override  TimeSignature #'extra-offset = #'(0 . 1.5) } 
        \context { \Staff \remove "Time_signature_engraver" }
        \context { \Score \override TimeSignature #'outside-staff-priority = #1 }
\context { \Score \override VerticalAlignment #'max-stretch = #ly:align-interface::calc-max-stretch
        \override RehearsalMark #'font-size = #2 %Studierzeichen Groesser
        \consists "Measure_grouping_engraver" }
      \context { \Staff \remove "Measure_grouping_engraver" }
        } 
%Takte!
global = { 
  \tempo 4.=116 \time 6/8 s2. 
  \dreizwozwoachtel s4. s4 s4
  \neunachtel s4. s4. s4.
  \time 6/8 s2. 
  \dreizwozwoachtel s4. s4 s4
  \time 6/8 s4. s
  \neunachtel 
  s4. s s  }

tempotakt = \new Staff   { \global  }
bassClarinet = \relative c { 
  \set Staff.instrumentName = "Bkl."
  \clef bass gis4. e4 fis8 ~ 
  fis 8 gis4 ~ gis8 e 4 fis 8 
  R8*9
  gis4. e4 a8 ~ 
  a 8 gis4 ~ gis8 e 4 a 8 
  R2. R8*9 }
  hornA =  \relative c'' 
  { R2. R8*7
    \times 3/2 { r8 gis } \times 3/2 {r8 bes } \times 3/2 {r8 g }
    R2. R8*7
    \times 3/2 { r8 gis} \times 3/2 {r8 bes } 
    \times 3/2 {r8 g } r4. r }
  hornB = \relative c' {
    R2. R8*7
    \times 3/2 {r8 dis } \times 3/2 {r d } \times 3/2 { r cis }
     R2. R8*7
    \times 3/2 {r8 dis  } \times 3/2 { r8 d } 
    \times 3/2 {r8 c } r4. r
  }
 Marimba = \relative c' { \set Staff.instrumentName = "Mar."
   R2. R8*7
   \repeat "unfold" 2{ \times 3/2 {<e a>8 r } }  \times 3/2 {<g c> r } 
    R2. R8*7
   \repeat "unfold" 2{ \times 3/2 {<e a>8 r } } 
   \times 3/2 {<g es'>8 r  } r4. r
 }
violine = \relative c''' {
  \set Staff.instrumentName = "Vlne."
  b8 gis4 ais4 b8
  fis4 dis8 eis4 eis
  R8*9
  b8 gis4 d'4 gis8
  cis,8 b' dis, eis4 eis
  R2. R8*9
}
  
\score {

<< 
  \tempotakt
  \new Staff \bassClarinet
  \new Staff { \set Staff.instrumentName = "Hrn.1/2" \partcombine \hornA \hornB }
  \new Staff \Marimba
  \new Staff \violine
    
>>

}