Center text over measure

[Urs Liska]

Hi all,

somehow I have the feeling this is a stupid question, but I'm unable to 
find the solution.

Is there a convenient and semantically clean way to center a markup 
above/below a measure?

I need a solution that doesn't rely on separate contexts (e.g. a 
Dynamics context above the staff). What *would* be acceptable is a 
Scheme function that calculates the actual width of the measure and does 
the centering from that. But since that would probably only be possible 
in a after-line-breaking callback it would seem quite dirty to move 
around a markup at that point.

Any suggestions?
Thanks
Urs