Well, it seems to work now.
Two files. oss.xml is the manifest and needs to imported to be recognized, like svccfg import oss.xml. Then there's svc-oss (derived from the rc3 one), the script doing the starting and stopping. Drop it in /lib/svc/method, chmod 555 it. If you put it elsewhere, you need to adjust the manifest.
You have to remove the hardlinks K89oss and S89oss from /etc/rc3.d after having set up the SMF service.
And I almost forgot, you need to enable it with svcadm enable svc:/system/opensound