Hey,
first of all, you should take a look at php autoloaders. Composer for example has autoloading functionality integrated and makes it pretty easy to define where your classes are according to their namespace.
This allows you to put your "normal" classes wherever you want. Oh and never use App::uses in your cake application ;)
- "Douche", containing the deliverymechanism-independent application) and
- "DoucheWeb", containing the web-application that uses the Douche-Application.
The classes in the Douche package know nothing about the DoucheWeb package (so in your case there would be no cake-dependent code).
The DouchWeb package is pretty small because it's only responsibility is to call the Douche application with the correct arguments and display the results as html. This is where your cake application could be. And your Controllers would use the Douche package to do their job.