Music spacing in incipits

[Rodolfo Zitellini]

Hello,
is there someone familiar with the code for the incipit in
http://lsr.dsi.unimi.it/LSR/Item?id=582 ?
The incpits work wonderfully - until I try to put in many pauses. Then
the spacing is just awful, all the pauses are far away from each
other. You can actually see this in the snippet where the few pauses
are far far away. Now imagine it with 12 longa pauses!
I was not able to find a suitable solution. How do I 'compress' the
music? I tried using base-shortest-duration but it does not seem to
affect anything in the incipit. Out of luck (and because I had to
finish a transcription) The only workaround I found was replacing a
notehead stencil with custom postscript to produce the pauses and
space everything using spacer rests:

tenorGloriaPause =
  #(ly:make-stencil (list 'embedded-ps
    "gsave
      0.3 setlinewidth
      currentpoint translate
      newpath
      1 0 moveto
      0 -2 rlineto
      1 0 rmoveto
      0 2 rlineto
        1 1 rmoveto
      0 -2 rlineto
      1 0 rmoveto
      0 2 rlineto
        1 1 rmoveto
      0 -2 rlineto
      1 0 rmoveto
      0 2 rlineto
        1 -1 rmoveto
      0 -2 rlineto
      1 0 rmoveto
      0 2 rlineto
        1 -1 rmoveto
      0 -2 rlineto
      1 0 rmoveto
      0 2 rlineto
        1 1 rmoveto
      0 -2 rlineto
      1 0 rmoveto
      0 2 rlineto
      stroke
      grestore" )
    (cons 0 10)
    (cons 0 0))

and then:
      \once \override  NoteHead #'stencil = \tenorGloriaPause
      c'\longa
      s1*19
      e'\longa


The result is beautiful but... this ought to be the least efficient
way of making an incipit!

rodolfo