How do you know the resorts are at 90% occupancy?? I don't believe this is true at all, maybe during holidays, peak time, etc, but I highly doubt the end of August. As some posters stated it also depends on the packages or special rates you're requesting. There are a specific number of rooms set aside for these codes.
We're going in the middle of August, a time that would probably be more crowded than the end of August, since the later in the month the more schools that will be in session. I just went on the official Disney site out of curiosity to check availability during the time we're going. Every resort I requested was available with all of the various room views. That's leads me to believe that there is plenty of availability and vacancies.
So, I really wouldn't be so concerned about the parks being so packed then. And, keep in mind, there are so many different things to do in WDW and also keep in mind there are many conventions that go on at many of the WDW resorts. Most of those guests won't be visiting the parks either. It's all relative.