Just heard back from the company. I'd like to share his reply because I've been quickly convinced it's perfect for me (and ManU might also be interested): The "Floor" of the upper, main compartment is on angle from front to back of about 30 degrees creating a wedge that forms the lower compartment. This lower compartment is accessible via a zipper that parallels the zipper for the front putter pocket. If you open that zipper, you can cram all sorts of items in that compartment. Since the dawn of disc golf bags, there never seems to be a big pocket where you can cram layers. This is particularly a hassle during spring and fall. I always make sure that our bags have a place to store large items like a hoodie or rain gear. You can definitely pack some gear in that space.
You can fit between 18-23 discs, depending on type of disc and orientation, while still having a jacket in the lower compartment.