You can "practice" a Cat II or Cat III approach any time an ILS is available that does not have an aotoland restriction. We do it all the time. However, as TopBunk noted, the pilot is responsible for alerting ATC that he will be attempting the autoland, so they can either protect the ILS critical area on the ground or warn the pilot that the area is not protected.
When "practicing" a Cat II or Cat III approach, you still cannot descend below the MDA for the actual approach in use unless you have visual reference.