Possibly it's only a starting point but I could use the files found at
http://www.pps.jussieu.fr/~letouzey/emul_debianppc.en.html to actually see I could boot some linux on qemu-system-ppc. Skipping the part aboud vde, I could boot it both with 0.9.0 and with a snapshot dated 20070406. From the documentation found in the links it looks like it used a patched 2.4.27 kernel and some other trickery, but, as I said it could be a starting point, and it's the only one I found :). Btw, it also seem to honour the -kernel option, i.e. without it doesn't boot... Hope this helps somebody. Lorenzo