I think.plums would do well. There is a small grove of plum trees that came with my place, under the shade of a couple big doug firs. They only get a little bit of sun all year, yet they produce delicious plums copiously. A little too well, in fact, I can never keep up with them in July/August.
It also depends on how tall the tree will get. It will reach for the light as it gets older, and when it's young it will probably be fine with the September shade; it'll just go dormant sooner I imagine.
I would aim for trees that ripen in summer, like summer plums and early apples (yellow transparent, for example). That way they have the sunlight for sugar when they need it. Trees developed for short growing seasons would also do well I imagine (St. Lawrence Nursery's stock, for example)
When you reach your lowest point, you are open to the greatest change.
As someone said - it's likely not such a big problem since the trees are going dormant then anyway.
I grow all my garden in the midst of a 50+ yr. old forest (very tall pines, oaks, madrones, fir trees) and nothing gets more than about 3 hrs. of sun a day in summer (some get nothing in winter because of a big hill to the south). I get wonderful apple crops, and Italian prune, asian pear, cherry, everbearing Illinois Mulberry, and fig crops. I planted english walnut, filberts, paw paws and a persimmon that have not gotten old enough to bear but are looking great. Some things have taken a long time to start fruiting - like two of the first apples I planted - took 7 yrs. but now give me wonderful crops. I have pomegranate, and hardy kiwis that are now starting to bear well, and grapes that bear well -but took 7 years to get started.
I just try stuff. Blueberries are my biggest disappointment - only the Sunshine Blue variety will give me any fruit to speak of. They must just need a lot of sun to bear. But I can't tell you all the 100's of types of plants I grow that experts say need "Full Sun" - that do just fine here. Some may not bear as well (or as young) as they would in full sun - but they do just fine. They give me "enough".... :-)