Holy shit. You know how some of the scene props (like stairs) are annoying because they face a certain direction and you can rotate them around, but can't flip them?
You totally can:If you set one of the scales to a negative number, it flips the model. The textures will be inside-out, but once you save and reload the map it'll look right again.
If you already knew about this, I hate you for not having posted it somewhere more obvious.