Switching between two- and three-staves piano score

[Thomas Ruedas]

Hi,
I'm trying to typeset a piece for solo piano where temporarily a third 
staff is needed. This is not uncommon in more recent piano literature, but 
I didn't find something suitable in the docs and archive. I did find the 
following snippet in the snippet repository:
http://lsr.dsi.unimi.it/LSR/Search?q=Adding+an+extra+staff
but it's not quite what I want. What I need is a full extra line in the 
score, potentially extending over a substantial part of the piece where 
it's used, but invisible/absent otherwise. Any ideas or hints for a trick 
on how to do that?
Thomas
-- 
Thomas Ruedas
Department of Terrestrial Magnetism
Carnegie Institution of Washington
http://www.dtm.ciw.edu/ruedas/