Guest Wireless Network -- Tomato 1.28 firmware on Buffalo WHR-HP-G54 Router

I'm not sure what the exact programming is on your Buffalo WAP but the first step is to set up the VLAN.  What kind of switches are you using?  Pick a vlan for guests, lets say 100.  Create it on the switch.  If the WAP is ONLY going to be guest and not for your staff as well, then make the WAP switch port an untagged member of VLAN 100.
Let's say you have a WAP on port 50, for guest only.
and a router on port 1
and another switch on port 25

e.g. config for switch

(config)#vlan 254
   name Guest
   tagged 1,25
   untagged 50
   exit

What I have is a small 8-port Asus GX-D1081 switch connected to the Buffalo router, so I effectively have 12 ports total, 7 of them occupied (including the one on the router to which the switch is connected).

I'm a little confused by your instructions. The only WIFI access point is the Buffalo router itself and the switch is connected to the router. Would I not set up the VLAN on the router --  by logging into the router through a browser window and setting up the VLAN through the Tomato firmware interface?

Since I am not connecting a separate access point to any of the Ethernet ports, and I want the only WIFI network -- the one being broadcast by the Buffalo router -- to be exclusively for the Guest network, can I just do that through the router's firmware? I would just basically be separating the Ethernet (cabled) part of the router from the wireless part and setting them up on separate VLANs -- wouldn't I?

Maybe the Buffalo router (and Tomato) just can't do that. I know quite a few newer wireless routers have the Guest Network function built in these days.  I was just hoping that it was possible to do the same thing with my trusty Buffalo router.

This answered every specific question I had. Thanks!