Thanks a bunch Mark,
That was very helpful indeed. I've been able to compile the sparc64-elf cross compiler, and that in turn did successfully build openbios.
Running Solaris with Qemu (3.1.0 and 4.0.0-rc0) now seems to kick off ufsboot and unix (sun4u) from the Solaris disk, and than crash into a panic stating: "bottom of 32-bit stack not aligned".
Does that sound familiar as well? Regards!