centering title and THEN adding something to the left of it

[Kieren MacMillan]

Hello all,

Please consider the following snippet:

\version "2.17.29"

\paper {
  bookTitleMarkup = \markup \fill-line { \abs-fontsize #18 \fromproperty 
#'header:title }
  scoreTitleMarkup = \markup \center-column {
    \fill-line {
      \line {
         \line {
          \line { \abs-fontsize #18 \override #'(box-padding . 0.5) \box 
\concat { "#" \fromproperty #'header:no } \hspace #0.5 }
          \override #'(baseline-skip . 0) \center-column {
            \abs-fontsize #18 \fromproperty #'header:title
            \abs-fontsize #13 \concat { "(" \fromproperty #'header:performers 
")" }
          }
        }
      }
    }
  }
}

\header {
  no = "1"
  title = "My Awesome Song"
  performers = "Me and You"
}

\score { c' }

I’d like to have the score title (i.e., the whole two-line column) 
automatically centered (i.e., it should be lined up exactly with the book 
title, in this case), and the boxed song number (which may have two or three 
digits in other songs!) “float” to the left accordingly. What’s the easiest way 
to accomplish this, without having to manually tweak each song?

Thanks,
Kieren.