LocalStorage has support of over 92% according to [caniuse](http://caniuse.com/#feat=namevalue-storage).
A lighter version of one of the [polyfills available](https://github.com/Modernizr/Modernizr/wiki/HTML5-Cross-browser-Polyfills#web-storage-localstorage-and-sessionstorage) could be included in the same way CryptoJS is. A very simple fallback on cookies could be considered as the value to read/write is not an object (no need to account for JSON compatibility!).

This is indeed only here for test purposes and (my bad, I hadn't realised) would break on IE7- and Safari 3.2- and Opera 10.1-. I'm not sure I understand what you mean by overriding "a method from the test used here" though. Do you mean adding a method to hawk.utils to read/write the LocalStorage? How to make the difference between a browser which doesn't implement LocalStorage and Node?