A bit of progress to report:

* Design updated to allow any combination of switching between the 4 modules

* Layout complete and sent off to manufacture!

Four layer boards are way more expensive than two layer. I think the design could probably have been done on two layers, but it would have taken me much longer. This layout is nowhere near optimal, and it took me long enough, so the time savings are worth the cash for now.

You can't see it in the picture but I put a couple of chips on the back to make routing easier too (side note: is there a way to make changes to the circuit in the layout and then import them back into the schematic? I should look into that...)

Next: start writing some Arduino software to control the switches on the hub, and some Raspberry Pi software to talk to the Arduino. (Yes, the Raspberry Pi could easily do it all but I don't want it to require a pi).