SOLVED (mostly):
There is a way to override the DetailView.tpl file for only a specific module, but be aware that the styling of the panels and actions menu do not appear if you do this and need to be added back in.
If you create yourself a file in custom/module/YOUR_MODULE/views/view.detail.php,
Then you need to basically copy the preDisplay() function from include/MVC/View/views/view.detail.php into your file.
You’ll notice on that last line:
$this->dv = new DetailView2();
$this->dv->ss =& $this->ss;
$this->dv->setup($this->module, $this->bean, $metadataFile, get_custom_file_if_exists('include/DetailView/DetailView.tpl'));
That it is passing the location of the main tpl file for the DetailView to the setup function.
I changed the file location to say:
$this->dv->setup($this->module, $this->bean, $metadataFile, get_custom_file_if_exists('modules/Opportunities/tpls/DetailView.tpl'));
And since thats wrapped with the get_custom_file_if_exists() function, it will go out and the file in the custom directory.