I think I understand what you're trying to do with data_object.php, and it's an interesting concept, but it makes all the code look and function unlike the rest of Moodle, and it's doubtful the rest of Moodle will change to use data_object in a hurry.

Unfortunately this new class design is making it very difficult for me to understand how your code works, which is a real impediment to getting it reviewed and included in Moodle 2.0.

At very least it should be in lib/completion, not lib/.

Can you write up a quick document explaining how your design works? Have you thought about scalability?