It turned out that last weekend I didn't specify the contest conditions good enough.
Now I have to tell that I've seen the boot prompt under qemu-system-sparc64, but being fair, it doesn't count yet, because
a) only Solaris 2.5.1 - 7 get it up to there. The goal for 64 bit machine is certainly OpenSolaris (or maybe "just" Solaris 10? What do you think?).
b) the system hangs right after printing the prompt.
The next goal is therefore a usable # prompt, not just the hanging one. :-) But stay tuned, I'm getting closer...