Header fields in custom footer

[Noeck]

Hi,

I am struggling with custom footers:
http://lilypond.org/doc/v2.19/Documentation/notation/custom-titles-headers-and-footers#custom-layout-for-headers-and-footers


Can I use a header property as url in \with-url?
\with-url \fromproperty #'header:url \fromproperty #'header:composer
does not work.

Can \on-the-fly only directly precede a single markup? I can't write
\on-the-fly #not-first-page \column \fill-line ...

How can I use \fromproperty in scheme? Because I want to do some string
operations before inserting the field in the footer.

Examples:
\markup \with-url \fromproperty #'header:url \fromproperty #'header:foo
\markup \on-the-fly #not-first-page \column \fill-line { "A" "B" "C"}


TIA,
Joram