Exactly. I'm doing this as well. I configured a second wifi interface, wl1, and once you have second interface in dd-wrt, you can apply bandwidth throttling. It works like a champ. I connect my systems to the primary wifi interface, and any guest can connect to the open secondary interface. To test the throttling, I fired up a bandwith test on the open guest interface, and then another on the primary network and confirmed that the primary network takes priority over any of the guest traffic.
I used this wiki to help configure my setup: