The "Rubric" grading method is an implementation of an Advanced Grading method. So your module would need to be gradable with an Advanced grading method. The only module that allows for that at the moment is the Assignment so you need to read through how assignment does it and implement support for it in your code.
My first steps would be to create an assignment that used Rubric, then grade it and trace through the code with a PHP debugger. This is not a trivial task, but doable.