This is supported in the Chrome, Firefox 50 and IE 11 debuggers and Closure compiler, and some other stacks have code to generate the maps.
The spec is at https://docs.google.com/document/d/1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6fAH0KY0k/edit?hl=en_US&pli=1# and a good overview is at http://www.html5rocks.com/en/tutorials/developertools/sourcemaps/ .
The minified file points to the source map with a line like:
or a header like:
If we use a dynamic URL, that should allow doing it in production. It would build the source maps on demand (just like the minification) for people that have them enabled (and are debugging), without slowing the site for anyone else.