it would be nice if the af points lit up when the subject was in focus. and then we could compose based on af point location and adjust the focus ring and stop when it lights up.
The AF points are deactivated and physically removed from the light path when the mirror is up in video mode. This diagram is with the mirror down, but you can see when the primary and secondary mirrors flip up (Live View mode), no light reaches the phase-detect AF sensors: http://leongoodman.tripod.com/d70/dslrchartweb500.jpg
As already stated, ML will give focus peaking assistance, however it's not available in a stable build on some cameras, and not available for the 5D3 running 1.2.1 firmware. This means you couldn't have clean HDMI out, or use cross-type AF point at f/8, or any other features and enhancements of 1.2.1.
One solution is using a clip-on LCD loupe to magnify the LCD. Several companies make these; I've used the Zacuto Z-Finder which is good.
Another solution is a field monitor which has focus peaking built in. Yet another is an EVF.
The Zacuto EVF Pro is my preferred solution since it provides another point of contact to stabilize the camera, shields your eye in bright conditions, is more private than a field monitor, plus has focus peaking and zebra exposure aids.
EVF Pro looks like what I need, but that cost is steep. If I start making real money from these videos i'll have to invest in one.