Can I set up domain filtering on an Airport Extreme?
Easiest way I know is to use a service like OpenDNS. You set up the Airport to send all DNS requests to OpenDNS and use their filtering (which can be quite sophisticated) to do the job of blocking or whitelisting web addresses. Of course you need to be able to protect the DNS settings on the Airport, but this is doable.

I have it set up for general filtering and monitoring on my home network which has more than 5 computers and more than a dozen wireless handhelds all served from an Airport Extreme.

Check it out at http://www.opendns.com/