Overall, looks pretty cool and would love to merge in this feature.

Specifically though - does this generate the less with media query source map and then extract out the info using regex?

Personally I would have changed env to allow each component to get a reference to an index marker as it does toCSS. Then after the less has output the css, I'd run another visitor to go through every node and extract out the source and target line information into an object that could then be output as a seperate source map.

What do you think?