Here is what I am currently thinking from what I have grown and what I have seen on other places.
All time favorite for Sept. hunt would be Chestnut crab. Chestnut has a long drop time. So does my rootstock carb which Stu and I think is Columbia and it drops some crabs from late Oct. right up until now. I also have some seedlings that are still holding one inch crabs that are probably ranetka from descriptions I have read. Some of those same seedlings have 1/4 inch crabs. Depends on the tree. some of Morse bunches are still holding 1/2 inch crabs.
Dolgo is great, but tends to drop all of it's apples over a short period.
Plant a whitney, dolgo, chesnut crab, then rootstock crab. Look at the wild crabs that hold apples late and try to graft a few. Trailman, kerr, centennial,wickson,and a few of SLN crabs are the ground and not producing, yet.
One farm client has a whitney that started dropping crabs in late July-during a year with an early spring. (It has some scion on it that I have permission to grab!) I also would like to try KinderCrisp.
Someone needs to plant 5of each rootsotck and about 10 of each seedling rootstock and see what develops in 10 -12 years time. I'll bet there are some dandy deer crabs that could be purchased at rootstock prices. There is a report on the Net of someone having 5 ranetka that held apples well into winter and size varied up to 1 inch. I believe they were on the Canadian border.
George-what are they using for crabs in cider out there? One forum member from just east of me says chestnut crabs make good cider and it may vary a bit with ripeness of the crabs.