I'm not sure about the license either, maybe it would be worth a shot to send them an e-mail and ask?
As far as the <i> element, we could easily implement FontAwesome without doing it that way. It would be easy to make it work with <span>, which has no semantic value, or we could just attach the necessary code to a specific class like "fa-icon" or something, and then we could add that class to any element as needed.
I've also already created a .less mixin that adds them using :before and :after, which is really nice because it means less markup, the only downside being that IE7 doesn't support :before or :after.
I will work on getting you a detailed list of places in mojoPortal that it would be ideal to use it, and what needs to change. I think most of the time it will just be a matter of changing from an <img> to a <span> with a specific class and a specific HTML entity inside.