I don't think you will have trouble getting
accommodations somewhere in September. I would expect to be at SSR or OKW, because they are the largest resorts.
I
do think you may have trouble getting a
DVC owner to rent to you at all because of the risk of last minute deals for the owner. Frankly, I would be very reluctant to rent to someone on such short notice. The problem is, once the owner makes a ressie for you, if your plans change at all, both you and the owner are really going to be between a rock and a hard place. If you change plans within the last 31 days, the owner could really be in a bind, and may not be in a position to help you at all.
IF you get anyone to rent a ressie to you on such short notice, I think you should expect to pay in full within a few days of the ressie being made, and you should expect the transaction to be completely non-cancelable, unchangeable, and non-refundable.
On the other hand, renters are not the only people who don't (or can't) plan ahead. DVC owners do too. Sometimes, OUR plans change and we end up with points expiring that we can't use. We have to either rent them or lose them, and we are really desperate. In that case, you could find somebody with October use year points expiring on September 30 who would be absolutely thrilled to rent the points to you -- and probably at a substantial discount.
So, you never know.
You might be well advised to check out making a cash reservation as a backup. If you can't find an owner to rent from, you still have a reservation. If you do find an owner to rent from, you can hopefully cancel the cash reservation with little or no penalty.
Good luck and have a great trip.