Use proportional spacing for a section in a score

[James Harkins]

What is the magical incantation to turn on proportional spacing in the
middle of a score, and then turn it off again?

The following is not successful in lp 2.16. Two measures are produced,
and they both use classical spacing. I would have expected the spacing
to be visibly different after setting the proportional spacing
properties.

\version "2.16.1"
music = \relative c'' { \repeat unfold 2 { c4 c16 c c c } }

\score {
  \new Staff {
    \music
    \newSpacingSection
    \set proportionalNotationDuration = #(ly:make-moment 1 16)
    \override Score.SpacingSpanner #'uniform-stretching = ##t
    \override Score.SpacingSpanner #'strict-note-spacing = ##t
    \music
  }
}

One section of my current piece features aleatoric cadenza-like
passages for two instruments, and the classical spacing does not look
good here. But classical spacing is exactly right for the rest of the
piece.

Thanks.
hjh