It's pretty simple actually.
First make a copy of a normal doodad or etc. for the upside down one (or alter the original if you don't need the right side up one).
Then hold shift and double click it to edit the Maximum Pitch Variation (holding shit is important, this allows for negative values).
For the value put in -3.14
Hold shift and hit okay
Now when you go to place it, the guide will show it right side up, but when you do place it, it will be upside down and you most likely won't see it at all.
Click it and hold ctrl while pressing page up to raise it up so you can see it.
Now remember that blizz didn't put any polies on the undersides of pretty much anything, thus you can see right through the underside and back of certain things. You should take care to cap off the top of upside down doodads so it doesn't look terrible where you can see right through half of it.