Turn a Mac Mini into a public WiFi hotspot

What I'd like to do, for an event, is use a Mac Mini to share a Internet connection via WiFi. However, I'd like to put up a splash page to the joining device's browser. It would display the free WiFi is sponsored by blah, terms and conditions are blah, etc.

How would I force that redirect on joining? I know plenty of companies do it but I can't find a solution that either isn't a Hotel level enterprise product or a hack to a existing router. Any ideas?

You could do it with LinSpot. They take a 15% cut of for-pay access. You can install their software on your Mac. http://www.linspot.com/

As for doing it yourself, it's a bit trickier. You'd have to run your own DNS server, so when a user connects to your network, the first request would be redirected to your splash page. Then all subsequent DNS requests would pass through your server to a real DNS server. That type of policy may be possible with OSX Server or iDNS server, but I'm not 100% sure on how it's done. You will need server administration software, and you will need capture the DNS to create your own Captive Portal (as it's called).