Wrote another hack, and now I can use OBP under qemu! Woo-hoo! All in all it took just 7 weekends. :)
The bad news is that.... It doesn't do much better than OpenBIOS. I can boot Linux, and NetBSD (this one is more complex, as OBP checks disklabel, and NetBSD miniroots don't have it), but booting Solaris 9 gives...
...the very error message as under OpenBIOS:
bootblk: can't find the boot program
I still think it is a progress: OBP has a debugger, while OpenBIOS doesn't. If nothing else helps I can step through the boot loader.
And anyway the effort wasn't useless: I found one bug in CPU, and three in scsi layer.