I had a similar idea when the CM first came out and revised it when the zero launched.
I'd do it like this:
- PCI or PCIe card with USB host controller and one or more USB hub chips.
- Multiple sockets for CM/pi zero with USB routed from each socket to a hub port
- PIs runing as USB ethernet gadgets
I abandoned this because:
- I'm not in a position to design the PCI/PCIe card
- With the zero you can get the same result just by pluging it into a spare USB port
- CM3 hadn't been launched at that time so zero was much more cost effective. It still is unless you really need the quad core performance.
- The clusterhat launched.
Incidentally, you don't need to fake a webcam to see the screen, use VNC over a network. If you really need to see the console boot messages, use the serial port either by adding USB to serial connections or by adding a serial header for each CM socket.