Centering textscripts

[Michael Krause]

Is there a way to center \f marks between the two staves in a
PianoStaff? I've tried various things, but the sign is always closer
to one of the staves (closer to the voice it is instantiated from).
If this is not possible, any hints where I could go hacking?

thanks for any help,

-- 
michael krause [aka raw style / lego] - www.tu-harburg.de/~semk2104/