The WRT54GS is exactly the way I have my network set up off the Set Top Box. All works well (untill I overload it with P2P stuff, but that's a different story
If you clone the mac address of a device previously directly connected to the STB (in your case the USB adaptor), then you wouldn't even get the STB provisioning screen to contend with, provided you reboot the STB after swapping over to it. On the other hand your browser will point to that screen the moment you try to use the internet, if you reboot the router, and don't clone the mac. As said above all you'll need is your PID no and password that should be on a letter sent to you when you took up broadband via STB.