Looks like we'd need another compilation process to render the source map, and output the additional .map file:
Would be interesting to add some of this to the UglifyJS plugin as well:
Code attached, although it uses data-urls to avoid a second compilation pass, so there's room for some improvement; perhaps an option to output separate .map files, but we'd need a clear way to be able to produce multiple output files from one input document - including the original coffeescript source, otherwise there's not much point to sourcemaps!
I've just merged the sourcemaps support and published master as v2.4.0. Give it a try!