Changing vertical staff spacing for PianoStaff?

[Matthias Kilian]

Hi,

is it possible to temporarily extend the vertical alignment of
piano staffs?  I've the problem that I need more space for some parts
only. Enlarging the distance for the whole piece would just look ugly.

Would it even be possible to let LilyPond automatically enlarge
the distance to avoid collisions between both staffs, i.e. use that
`forced-distance' as default but use a larger value when detecting
vertical collisions?

Regards,
        Kili