I can't see any obvious way to do this using the parts you've listed. If you had a second AirPort, bridge mode could take that guest wireless network and share it over ethernet. There are also wireless bridge devices that can turn a wireless connection into ethernet, or several ethernet ports if combined with the switch.
There is one thing you could try. Connect the modem to the switch and the switch to the router. This usually doesn't work, but some internet connections allow you to connect more than one device (via a switch) to the modem. If you can connect another computer to that switch and have it get an IP address while the router still has access with its own IP, you may have a solution. The guest's PC will be outside the router and have just as much access as any other PC on the internet. This is a long shot, but it won't hurt anything to try.