two things:
1. if you Google parking and the centrale station milan you should find that the station has a garage and it is across the street from the hotel.
2. more interesting to me is that in your post you list the garage palace at the piazza della republica. this is about a 6 to 8 block walk from the meridian and more importantly is exactly where the westin palace is located. i have stayed at both hotels and the westin palace is much better than the meridian and in a better neighborhood ( the area around the train station is pretty creepy at night). if you could move to the palace you would have a garage right there for sure.