Both my auto_iso module (in M mode for EC) and the autoexpo module work by calculating the Bv from reading the AE value.
Unfortunately, this doesn't seem to be immensely reliable, at least on my cameras (60d, 6d is even worse) the AE reading can overflow and also very often returns values completely off the scale, causing a metering lag since ML has to wait for a correct reading and potentially causing mis-exposure when the lighting changes quickly... currently I couldn't really recommend this mode for critical shooting, while auto_iso in Av/Tv naturally always falls back to a correct Canon exposure setting.
Pravdomil mentioned a potential possibility of directly reading the metering w/o the AE problems, is this an actual possibility? Sorry if this is a dupe, is there has been any further discussion on this I'd be grateful for a link to it.
http://www.magiclantern.fm/forum/index.php?topic=8007.0