Several folks who I am cruising with in August were able to book the Entertainment rate for various dates the week of Aug 10-17, so YES, you can get Entertainment rates in the middle of the summer.
I'm pretty sure they were able to book rooms farther than 360 days out but you had to call the hotel directly.
Edited to add: they booked directly with the hotel at a group rate, but when online booking for Aug '08 became available in Aug '07, they were able to switch to the Entertainment rate. Some called and had the reservation changed. Some booked a new reservation online with the SD50, then called and cancelled the original group rate reservation.