My thought is that this is really most useful for CSS and the transformations that SASS/LESS are capable of are relatively mundane in comparison to CS -> JS. In addition I only see this as a tool for development, not something that I would want running in production so you don't have the concerns over increased file size or still supporting minification that you do with source maps, assuming you want source maps in production for error reporting.
I can envision simply using structured CSS comments to pass the needed information to the browser. The browser only needs a structured pushing protocol, baseline comment format, probably handling of comments that indicate file boundaries (used for pushing to the correct resource) and a 'plugin api' to allow plugins to parse other comments in a meaningful way, SASS/LESS could provide a 'Dev tools' plugin to parse the comments and support whatever editing features they wanted. They would also have to support generation of the comments in the preprocessor.
The important bit is a standardized pushing protocol, a standardized way to provide additional mapping information and a standardized 'plugin' style system to allow things like SASS to add to dev tools additional editing support.
Just one theoretical approach, still a ton of work, but I really think there is a lot of developer time spent dealing with this issue so it may well be worth it.