Source Map Specification Discussion Mailing List
If you are interested in discussing the source map specification and any future modifications to it, please join us on the new dev-js-sourcemap mailing list. The Google Groups archive hasn't synced yet, but I am told that it should be synced within a week.
This list exists only for discussing the source map specification, not any of the various tools which use source maps. They each have their own preferred mediums of communication which you should use for tool specific discussions.