Custom key signatures

[Kees van den Doel]

I managed to print custom key signatures from a microtonal font. One problem 
remains, and I wonder if anyone can help me.

In my init file persian.ly I have defined variables for various key signatures 
and keys, e.g. for Shur in E

shurEKey = \markup{
  \translate  #'(0 . 1)   \SORISYMBOL
}
shurE = #`(
    (3 . ,SORI)
  )

were SORISYMBOL is my UTF-8 code form the font I've loaded.

My source file looks like this:

\include "persian.ly"
\score {
  \relative c' {
           \set Staff.keySignature = \shurE
           a b c
     }
  \midi { }
  \layout { 
    \context {
      \Score \override KeySignature #'text = #shurEKey 
    }
  }
}

which works fine. I would instead like to put the statement

 \Score \override KeySignature #'text = #shurEKey 

not in the layout{} but in my music expression, because now I can't change key 
signature in a piece.

If I try that it will complain that shurKey is not a markup.

Can anybody explain what the problem is?

Thanks,
Kees