I want to designate to certain people the centerpieces at the table. Would it be ok to mention it to the persons we want to have the centerpieces a few days before the wedding just in case we forget to mention it to them then?
Yes, but these may be taken by someone else anyway. Sometimes people just think it is there for anyone to take home. At one reception I attended there was a star under the dish designating who would receive the centerpiece. So, you could seat these people as special and place the star, or whatever, at their place.