We made a reservation for DS's first haircut in January. I didn't make it at 180 days, but maybe around 90, but I'm thinking that they do allow you to at 180 days. We didn't have any problem getting in.
The biggest thing that happened to me is I called to make the reservation (called the dining number and the number that was on the website for making HBS reservations) and they told me I had called the wrong number and that they had to transfer me. Then I talked to someone else and they said "Oh you need to make those reservations with dining" and they transferred me back. So I think it's still a little new for dining to take these reservations and sometimes you get a CM that isn't aware that it is allowed.