Sorry for the delay in reply. I do plan to get to debugger support. I just haven't had a lot of time to work on mirah lately (real job & family life, etc...). I don't think it should be too difficult. Ultimately it is just compiling to the same java byte code as the java source. I just need to tell netbeans that :)
But I don't have any helpful suggestions at this point for the implementation. My plan of action would be to check the netbeans source to see how they do it for java, then see what needs to be done to make this work for Mirah. If you want you can add an issue to the issue tracker.