If the system actually follows usb3.1 specs 900mA is the max you'll be able to draw, since unlike the usb2.0 specs there is a max current defined. With 2.0, it was defined as supplying at least 5 units (100mA per unit) and many manufacturers allowed much more than that. Are you actually going to be using usb3.1 and superspeed though? Most boards that use usb-c just run it in 2.0 mode, and I don't think the atmega series can handle 3.1 natively, so if you want 3.1 support your hub will have to handle all of that. I'd certainly like some info either way on your hub plans, I'd love to add a hub into one of my designs but I haven't looked into designing one at all.
As for power draw measurements, if someone else hasn't taken any for you by the end of this weekend I'll see if I can get some. I've got a filco tkl with an HID Liberation Device on qmk, it uses a 32u4.