How to change clef style

[Morten Lemvigh]

I'm trying to typeset a book in different styles; modern and ancient 
looking.

By including the same file in all the pieces I've almost succeeded in 
making the ancient style, with something like:
\override NoteHead #'style = #'neomensural
in a layout block

But I cannot figure out how to change the clef symbols. I just need to 
change the symbols to something ancient looking. It shouldn't be a 
direct clef change, I would like the notes to stay on the same lines in 
the staff, as if it was still a G- or F-clef.

Can this be done in some clever way?

Regards,
Morten