At the Velocity Online Conference today, we announced the option to collect above-the-fold (AFT) render time on webpagetest.org (WPT). This is an option that runs in tandem with video capture to analyze screen shots to identify the time/frame at which browser window visible content is rendered.
To try this out for your web page, click the option on the Video tab at the bottom of Advanced Settings.
It should already be exposed in the XML interface (probably as in the page data section). The XML dumps all of the data fields automatically. It should be easy enough to add as a metric to WptMonitor. Just be aware that measuring AFT is FREAKISHLY expensive because it has to capture 4 minutes of video. That means the test machines will each be tied up for 4 minutes per url being measured.