The reason there aren't many keyboards with USB hubs is power - a keyboard and one extra device might work, but beyond that you would have to specifically choose low power devices.
Most USB hubs also connect to mains power to get round this, so if you're building a keyboard that will not be moved often you could use one of them? Any hub will do, usually the smaller the better so you don't need a huge case.
Or if you have a USB3 port to connect it to a USB3 hub should work as it has much more power.
