Well, for the first time EVER i'm going to contribute to a thread with MEANINGFULL and NON SPAMMY response.
Example : You are in 5th gear. And you need 15% throttle to maintain a 60mph cruise on the straight and level.
Surley a program , probbably Via insim would be able to make LFS think you have got the accelerator pedal held down 15%. Then, and here comes the more complex part, when you go up/down a hill, a couple of mathmatical equations calculate the force (gravity [from the hill, whether its going to slow you down {uphill}, or speed you up {downhill}]) and will add more throttle, or decrease throttle accordingly.
Like, in a real car (afaik) the cruise control will just use the electronic throttle to maintain at xx (or xxx) mph based on inclines/resistance and so on
After you figure out the calculations for the up/downhill, i'm sure it wouldn't be too hard to do.