I splitted the patch.
It was tested with an old Linux distro (RH 4.2) and with NetBSD. The SMP kernel must be selected during install.
Here is, for example, a NetBSD/sparc32 image with SMP enabled :
qemu-system-sparc net702.qcow -M SS-20 -vga cg3 -smp 2
This SMP mode used to work a while ago, but it has so far no practical purpose as enabling it slows down QEMU considerably.