Well, I hope this trick hasn't already been discovered, but here goes:
I found you could make a nice, raised-looking 3D bridge that seems to have a rocky pillar in the centre. No real trigger tricks involved, just a nice map-design thing. To see how it works, just delete the bridge sections. I think this would be appropriate for any Osgiliath scenarios. Enjoy!
P.S. Sorry about not having some sort of tutorial, I may implement one in the future.