Well its a lot of work... My suggestion:
Get a model that big (or scale it up) but to be very High Quality, then make it a Bridge (on which it would be possible to walk on) and raise it as high as posible, then some pathing blockers at the ground for the boundaries... The other is up to you then.
Also i think you should put a really high amount of Fog down down there, or some Image... That's how i would do this.
- MY QUESTION
Is there any possible way to create your own 'sky box'... because you could put it just above the tarrain and make it look like you're on a mountain or whatever...?
[I am making half-life maps too and i know that it is possible on valve hammer editor but it's a different engine so i am just guessing]
EDIT:
I thought a bit of what I've said and I think that if you raise the doodad (bringe), the camera wont react on it and it will stay under the arena... so if you maybe raise the terrain just under it so that it wont be seen you could fix that.. Also if the sky box is posible you could just make it near the edges of the arena and it would hide the terrain under it...