Booklet from Docbook

Dear all,
this is a XSL-FO related issue but some of you might be able to provide
me some hints (I allready posted this on XEP support. So sorry for
crossposting): 

I'm currently trying to build a booklet (2 A5 pages on a A4 landscape)
from my DocBook sources. I made some sucessfull experiments with psbook
and psnup. In general this works fine, but I need to put a large
overview-graphic in the middle of the booklet which spans the whole A4
page. 

1) Is it possible to build such a document by using XSL-FO?
2) How can I tell the PDF-Renderer to put the graphic in the middle of
the booklet (position may vary)?

Thanks for your help. Regards.
Ludger Thomas