Nagios Check that Simulates Web Full Rendering Time

Selenium is the defacto standard for full-browser-render testing, so take a look at the check_selenium plugin

Could you script a browser? You might be able to use AppleScript to tell Safari to load the page and time it, if you are running on a Mac.

I don't know if this is close enough to a 'real' browser for your purposes, but another option is to use HTMLUnit, which is a Java library that is used for things such as User Acceptance Testing, Screen Scraping, and so-on. You could use the same library to time how long it takes for a page to load, but obviously this is a Java engine rather than a real browser so the performance may not be representative of what any particular user experiences. You may also not want to run Java-based Nagios scripts.