(docme?) should \fromproperty work for any \markup?

[Graham Percival]

In the following example, \fromproperty #'header does not return 
anything.
It works correctly when called from inside \paper{}  (ie for 
constructing
titles).

Is this an undocumented feature (in which case I'll document it), or a 
bug?


\header { piece = "foo" }
\markup {
  between
  \fromproperty #'header:piece
  after
}