@Munter CSS imported via the official plugin (https://github.com/systemjs/plugin-css) 0.1.20+ uses the asset list feature we came up with, and should work out of the box. Templates are solved "in theory", but requires collaboration with a plugin. @gustavnikolaj wrote one for knockout templates, should be adaptible.
They need to emit an asset list. It's an array of items that can either be a url or the actual source + source map.
It's pretty much one-to-one with our asset config objects.
There's no specific support for images, but again, it's solved in theory. Until plugins emerge, you can still use GETSTATICURL in the code that will end up being bundled. I would very much like to replace all that with a system.js plugin.