In a composer.json the extra node is reserved for third-party applications to put their stuff in. Third-party devs should be able to plug-in their parsers into composer to provide a nice API on their own, instead of the JsonObject and JsonArray types.
The API should be able to provide two endpoints.
ComposerPackage composer = new ComposerPackage(new File("composer.json"));
MyDomain domain = (MyDomain)composer.getExtra().getAsObject("node");