Google provides a variety of \'cards\' for Google Now (http://www.google.com/landing/now/). Is it possible to create your own cards? The system looks pretty modular, but I haven
Note quite an answer, as it is still not possible to create Google Now cards, but you can now hook into the Google Now search function (basically Android's Siri) and provide custom search results. For example you can say "show me the lyrics to..." and it opens a lyrics app.
Here is a link to the project which is based on the Xposed framework.
Just guessing from my impression of the Google Search apk (which includes all the Google Now functionality and even the home screen on KitKat), it should be possible to use a similar technique to inject cards into the app - however since the app is huge and very complicated, it will be a lot of work. I'd keep my eyes open on the xda-developers forums, wouldn't be surprized if someone there solves this in the future.