I think you missed the following point:
Version 3.5 gave access to the HomeController instance from a plug-in instance. Then, from the HomeController instance you can access to the other controllers and their views if necessary.

If exportToOBJ method doesn't perform like you want, I don't see the problem to write your own method in your plug-in. But if you want to change the behavior of the existing exportToOBJ method, you'll have to create a derived version of Sweet Home 3D, not a plug-in.