This would be nice to have. JavaScript is becoming more and more of a serious language, and it would be nice to have the SquirrelFish interpreter available as an alternative to SpiderMonkey, Rhino, and others (especially if it's really that much faster!).
JavaScriptCore is already available as a standalone framework or library. The source comes with a simple application currently named "testkjs" that provides both an interpreter that will execute .js files and provides an interactive prompt. As far as I can see, this feature request is already fulfilled.