Why not just do this literally with the sun angle and atmosphere adjustments if necessary? Terragen's sunlight and atmosphere modeling is quite good, so you can get some very nice, realistic "light crawling down/up a hill" effects for sunrise/sunset. Just set your sun angle very low, maybe 1-2 degrees, adjust the sunlight heading to be coming from the right angle to strike those mountains, make sure there are no other mountains in the way, and then if the colors aren't right start experimenting with the atmosphere settings, maybe increase redsky decay, or shift its color a little.
- Oshyan