Full disclosure: I am the cofounder of earnmojo.com
There are (at least) two ways to address the engagement/loyalty problem with a third party awards solution: 1) provide a simple, drop-in JS (ala Google Analytics) or 2) provide deeper, API-level integration.
We've started with the first approach, which won't work for everyone, but it has worked for a lot of non-technical people, many of whom have set Mojo up on their sites in < 30 minutes.
For our friends on HN (and other developers), we have a private API. We've used it internally a few times, including rolling out achievements on the conference web site for JavaOne and Oracle Develop. We intend to publish it soon, but if you want early access, just let us know.
chris@earnmojo.com, @bucchere on Twitter or 415-516-3941