From my brief research of the Evernote Developer API docs, and the existing plugins I use, there seems to be no existing way to develop plugins for the desktop and mobile apps.
Apparently all one can do is create their own GUI and integrate with Evernote's data API or create a browser plugin that can be used with Evernote Web.
Is it possible to create a desktop and/or mobile Evernote GUI Plugin?
Evernote does not currently support development of Evernote Client application plugins for desktop or mobile.