Is it possible to augment editing features of Evernote? I was looking through the topics on their dev website and only saw interactions with entire notes or notebooks.
Evernote doesn't offer any in-client plugins or other means to alter the editor or client app behavior. If you need something for your own purposes, you might be able to implement some customizations/logic on top of Evernote web client by using a browser extension (which has access to the page DOM and can inject custom scripts). This approach is pretty brittle, though.