Yeah I had a symmetric building and the easiest way to make it was to just mirror the hallway on the other side. I definitely think supporting negative scale is worth it for this kind of thing. I'm probably just gonna change the face culling mode depending on the matrix determinant thing.

I would think it would be better to have a material attribute to change the behavior. For instance, a cloth might be scaled normally but still want to have a back face.

You could use that as well for the mirrored objects.

Edit: Then again, maybe this is not the best way because everything with that material will be double sided...