How to center custom dynamics marks between piano staves?

[Matej Kosik]

Hello,

I have put Lilypond to test to see if I will be able to engrave one
particular piece I like. It is a Hungarian fantasy

        Cserebogár, sárga cserebogár

written by

        Mihály Ignác

Here is the result.

http://altair.sk/mediawiki/upload/f/fa/Mihaly_Ignac_Cserebogar.pdf
http://altair.sk/mediawiki/upload/8/89/Mihaly_Ignac_Cserebogar.tar.gz

I have old printed copy but the paper slowly breaks into pieces because
it is fragile in a funny way.

There are some (mostly) minor issues that I would like to solve.

My first question is related to dynamics. At the moment I can center
dynamics \p, \pp, \f, \ff etc marks in between the staves according to
piano template, I can put these mark in the space in between the staves.

However, I would like to put there also other information defined as
follows:

        morendo = \markup{\large\italic morendo}
        crescendo = \markup{\large\italic cresc.}
        rall = \markup{\large\italic rall.}
        ritardando = \markup{\large\italic ritard.}
        etc.

At present, I position these marks below appropriate notes for the right
hand. I would like to put those marks to the place (Dynamics) reserved
for such symbols where I already put

        \p
        \pp
        \f
        \ff

Is that possbile? If I do that, I get:

If instead of

  \parallelMusic #'(upper lower dynamics)
  {
    ...

    \barNumberCheck #5

      d32\ritardando(a d, a')
      d(a d, a')
      d(a d, a')
      d(a d, a')
      d16_\morendo[a8.^^] d16[a8.] |

      r1 |

      s1 |

    ...
  }

I would like to write something like:

  \parallelMusic #'(upper lower dynamics)
  {
    ...

    \barNumberCheck #5

      d32(a d, a')
      d(a d, a')
      d(a d, a')
      d(a d, a')
      d16[a8.^^] d16[a8.] |

      r1 |

      s4\ritardando s4\morendo |
    ...
  }

but I get an error message:

  error: syntax error, unexpected MARKUP_IDENTIFIER
        s4
          \ritardando s4\morendo |

What is the correct way to engrave those dynamics marks centered in
between the staves?

Thanks in advance for the help,

Best regards,
--
Matej Kosik