We don't have any standalone javascript packages in the tree at the
moment but I know there's been some interest before. Is anyone still
(planning on) working on javascript stuff in-tree?
Not in tree, but I was planning on an idea called enpm but I realised resolving dependencies in npm is horrible, almost as bad as the live code on building with Go. The only way I could see this work is to package the dependencies with the app in a single tarball similar to what Vagrant does now. It is not the Gentoo way but these new systems (gem, Composer, even Go) seem to not care very much about having packages you can consider stable, nor predictable dependencies.
I would still find it useful to install CoffeeScript (among others like PhantomJS) via Portage for global use. Right now I hack on ~/node_modules/.bin to PATH in my shell (luckily that works).