Sorry to resurrect this topic, but I am looking for answer to this same question.
I echo’d out $this->dv and saw there is an array value called ‘[tpl]’, that references include/DetailView/DetailView.tpl.
I tried to add this line into my module’s detailviewdefs.php file but got no luck redirecting it to a custom directory. I see guides for how to do this with the header.tpl and footer.tpl files but not the DetailView.tpl file.