Install of OpenSolaris itself went well, but network was not available.
Searching around the net led to
http://kb.parallels.com/de/7060 (or English: http://kb.parallels.com/en/7060)
But still no avail.
It turned out that the script change is almost complete, but is missing
one additional line. Below "Compiling driver ..." it should read
cd $tmpdir/$driver && /usr/ccs/bin/make install > /dev/null
After this change
dladm show-physshows the interface and
ifconfig ni0 plumbis able to use it. After a reboot, networking is working :-)