It occurs to me that the cisco wireless routers, or the linksys wired router, might have that very feature already built into their firmware. Something that redirects someone to a preset web page the first time they connect. Have a peek through the documentation of those devices and see if it's possible. You might be able to do this without leaving your desk!

Is that a feature of Tomato by any chance? Can you put Tomato on the Linksys router?