Internet and Web Sharing URL Redirect

What you're asking for is actually a lot harder to do than you might think.

While your Mac is acting as a gateway device for the connected systems, it isn't do much more than passing packets from one side of the network to the other. It doesn't care about the contents of those packets, or what protocol they relate to - a packet comes in, it gets NATted and forwarded on.

In order to do what you want your server has to first identify that the traffic is, indeed, web traffic (you can't redirect other protocols such as mail, chat, etc.), and you then have to verify whether the user has already authenticated or not - you only want to intercept/redirect the users who have not yet authenticated, otherwise even the authenticated users would end up getting redirected to the authentication page, in a circular loop.

The easiest solution is probably to install a proxy server that requires authentication but you still have the issue of configuring the clients to use the proxy (the DHCP server can be tweaked to hand out proxy server information, but the client is not obliged to honor it).