An alternative: use an embedded Common Lisp and connect to it via SLIME from Emacs. Thus you could build an environment in Emacs for controlling the App and download/executing Lisp code inside the App.
This is a fine way to work. I, for one, love SLIME. However, the main goal of Emacsy is not to load code at runtime; it's to be able to modify the UI of your application at runtime while providing some Emacs-like facilities.
Certainly, the normal way of embracing an application with Emacs is to turn Emacs into your frontend as you suggest. That works well for a number of applications, but it's not something you can distribute as a complete application, instead you might include your-app.el and hope for the best. Emacsy provides a way for you to improve your actual applications UI. Moreover, it allows your users to modify the UI to suit themselves.
