Centering multi-measure rests

If you want to centre multi-measure rest numbers between the two staves in a piano staff, a custom context can help.

\version "2.24.0"

\layout {
  \context {
    \name PianoMMR
    \type "Engraver_group"
    \inherit-acceptability PianoMMR Dynamics
    \consists "Axis_group_engraver"
    \override VerticalAxisGroup.staff-affinity = #CENTER
    \override VerticalAxisGroup.nonstaff-relatedstaff-spacing =
      #'((basic-distance . 1) (minimum-distance . 1) (padding . 0.5) (stretchability . 1))
    \consists "Multi_measure_rest_engraver"
    \hide MultiMeasureRest
    \override MultiMeasureRest.Y-extent = #empty-interval
    \override MultiMeasureRestNumber.Y-offset = #-1
  }
  \context {
    \PianoStaff
    \accepts PianoMMR
    \override VerticalAxisGroup.staff-staff-spacing.padding = #14
  }
}

musicOne = {
  \clef bass
  \key f \major
  \time 3/8
  \partial 8
  r8 | R4.*7 |
}

musicTwo = {
  \clef bass
  \key f \major
  \time 3/8
  \partial 8
  r8 | R4.*7 |
}

\new PianoStaff <<
  \new Staff \with { \omit MultiMeasureRestNumber } \compressMMRests \musicOne
  \new PianoMMR \musicOne
  \new Staff \with { \omit MultiMeasureRestNumber } \compressMMRests \musicTwo
>>