I am assuming that this 18ft high ceiling is flat, in which case lighting from the walls (I'm imaginig tracks run on the walls vertically to cover the whole height evenly) would create a dramatic effect. probably light it better than from the ceiling. My concerns would be getting the proper beam spread. Probably narrow spot because of how far away the head will be. I like the MR-16 light quality.
Another concern I'd have is trying to conceal the light source. It would look better if the heads where hidden behind a beam or something. Just having them running on the wall might distract the view.
The mantel idea would give it an enirely different look. You would have shaddows because of how close the light would be. Some might like the effect, others may not.
Once, when I was asked to light the facade of a building, my supplier got me in contact with a rep. from the manufacturer. They entered all the distances into a computer program and it spit out which fixture with the proper wattage and did a point to point diagram showing light levels over every squre foot or so. Knowing how to interpret those numbers gave a good indication of what it would look like showing hot spots and such. The reason I mention this: maybe get in touch with a lighting manufaturer (Juno or the like) and see what they say. I was pleasantly suprized at the level of service (and it was all free). My experience was with Lithonia (obviously a different application.)
Manufacturer for jam switch: I've used Edwards
