There is already a Tapatalk client for Harmattan: Siilihai

I also made my own one: Tapasboard. I never got to "market it" because Siilihai works fine. Obviously, I like the interface on mine better

You do not really require an API key if you don't want to use the "push notifications" features, which are useless because they are closed source. I contacted them about this and they made clear they have no intention to make push notifications available to 3rd party clients, so I'm actually against enabling them in TMO.

(In fact I once considered forking the spec to add true universal push notifications support)