Using Scheme inside header markup

[Sven Axelsson]

Lilypond 2.6.0 Windows

It feels like I've tried everything, but I can't get the s
yntax right for this.

How do I use Scheme in header markup? I want to turn some 
fields on and off depending on some external conditions. 
I am trying this:

myCond = ##t
\paper {
    bookTitleMarkup = \markup {
        \column {
            \fill-line {
                \large \bold \fromproperty #'header:title
                \fromproperty #'header:meter
                \line { \fromproperty #'header:composer " " \italic
\fromproperty #'header:arranger } 
            }
            #(if myCond ( #{\line { \fromproperty #'header:comment } #} ))
        }
    }
}

\header {
    title = "Test"
    meter = "Test"
    composer = "Test"
    comment = "Test"
}

\relative c'' { a b c d }

and various variants of the same, but it always gives me a 
"unknown escaped string" for all the backslash expressions
at the point right after \header, and "unexpected SCM_T" at
the #(if statement.

Surely this is supposed to work?

-- 
Sven Axelsson