Tight spacing in mensural notation (was: Re: Cadenza Senza Tempo Problem)

[Graham King]

"...and most important you want tight space between all notes as
though it's not allowing the space for barlines and more."

This unanswered part of Reggie's question in [1] lead me to re-scratch an old 
itch.  In manuscripts and old printed editions of mensural notation, notes and 
rests are horizontally densely-spaced without regard for their musical 
duration.  I have struggled to reproduce this in lilypond, and the nearest I 
can get is this:

\version "2.19.82"

\new Score \with {
  \omit TimeSignature
  \override SpacingSpanner.spacing-increment = #0.0
  \override SpacingSpanner.packed-spacing = ##t
  \override NoteHead.style = #'petrucci
}

{
  \cadenzaOn c'\longa \breve 1 \breve 1 2 2 1 4 4 1 2
  % \bar "" \break % uncomment to observe effect
  2 8 8 8 8 c''1 b' a' g'
}

It would be good to be able to impose a small amount of horizontal space 
between the noteheads, but I'm at a loss to see how that can be done.  Any 
ideas?

-- Graham

[1] https://lists.gnu.org/archive/html/lilypond-user/2019-10/msg00420.html