overriding text alignment in stylesheet

[Kieren MacMillan]

[Lilypond 2.11.42]

Hi all,

I'm trying to define an orchestral stylesheet, based on the  
Barenreiter NMA soft-bound edition (Mozart KV 551, to be exact).  
Their instrument names are centered in a column, which presents no  
problem; however, the InstrumentName text itself is a center-aligned  
column if there are multiple lines in the name, e.g.

    Violoncello
       e Basso

In my stylesheet, I'm trying to set this so that the \header block of  
a piece which \include-s it will not have to explicitly \center-align  
the name. However, while the first two \override lines in the  
following code work as expected, I can't find anything correct for  
the last one (to force the column of text to be centered):

\layout
{
        \context
        {
                \Score
                \override InstrumentName #'font-shape = #'italic
                \override InstrumentName #'baseline-skip = #2.2
                \override InstrumentName #'self-alignment-X = #0
        }
}

Any hints would be appreciated.  =)

Best regards,
Kieren.