Websense's Advanced Classification Engine (ACE) will analyse links that users click on in real time. If it considers them to be dangerous the page will be blocked and a warning message will appear. Reckless users may still choose to click through to the site.
Testing pages dynamically is an interesting approach. Many similar types of systems, such as those offered by desktop anti-virus programs, use website reputation rather than looking through the content each time the site is visited.