In the Object Editor you have it: Art - Maximum Pitch Angle.
Shift + double-click will allow you to insert negative values which are necessary. They are in radians, although it says degrees.
-1.57 radians = -90 degrees, a tower/obelisk will lay on the ground.
-3.14 radians = -180 degrees, the doodad is upside-down.
Be careful though. Most models are one-sided. So when you turn them upside-down (for example a dalaran tower) you will see that their bottom is a hole. From inside, the "walls" of these models are invisible.