-> setenv boot-command boot

same result (light grey screen)

->boot hd:,\boot.img BD=MorphOS

same result.

-> dir hd:\

http://obligement.free.fr/temp/powermacg5_dirhd.jpg
(I have reinstalled MorphOS on the first HD. So now it's a fresh defaukt install, without Mac OS X, and the partition system is named "System")

-> Did you see your HD when running diagnostic-CD?

Yes, two hardrives (160 GB and 1000 GB) + the DVD reader:
http://obligement.free.fr/temp/powermacg5_asd_massstorage.jpg

-> Booting from the Open Firmware

"DO-QUIESCE finished" message and nothing more.
http://obligement.free.fr/temp/powermacg5_bootmacppc64.jpg