In my humble opinion, if you do a practice CatII/III into a CatI field, (especially without experience with that specific ILS) you could find yourself in a situation where you realize excactly why it's only CatI...
I don't know much about the criteria for II/III approval, but I do know the accuracy is vital ;-) As I see it, there's two options in regards to practice approaches; be very prepared to do a go-around, or be very prepared to disconnect and hand-fly.