\version "2.21.0"

cadenza = \relative c' {
  \override Beam.auto-knee-gap = #0
  c8[ c'' d,, b'' e,, a' f,] |
}

solo = \relative c' {
  c8 d e f g a b c |
  \cadenza |
  c8 d e f g a b c |
}

others = \relative c'{
  ees8 f g aes bes c d ees |
  %\scaleDurations #(skip-of-length cadenza)
  % How can I scale _this to the cadenza length?
  {f4\fermata r r2 |}
  ees,8 f g aes bes c d ees |
}

songStructure = {
  s1 |
  \cadenzaOn
  #(skip-of-length cadenza)
  \bar "|"
  \cadenzaOff
  s1 |
}

tacet = {
  R1 |
  <>\fermataMarkup
  % Why doesn't _this MMR show up?
  #(mmrest-of-length cadenza)
  R1
}

soloPart = \new Voice <<\songStructure \solo>>
othersPart = \new Voice <<\songStructure \others>>
tacetPart = \new Voice <<\songStructure \tacet>>

<<
  \soloPart
  \othersPart
  \tacetPart
>>