I've posted a few threads here to better understand the issue of setting the exact height for a grid point "in-world" using a script. Unfortunately, it appears that the only script function provided at this time for adjusting terrain height is "llModifyLand()", which is essentially a sledgehammer approach -- it acts on a set of grid-points, and adjusts the height, smoothness, etc. relative to where the terrain is. It does not provide the means to set the exact absolute height in meters to a particular x-y grid point, keeping the adjacent grid points completely untouched.
So, assuming my assessment is right (no one has yet pointed me to an existing function to do this), I went over to libsecondlife.org and put my money where my mouth is, offering a bounty of L$15,000 for someone to write this function (which I tentatively called "llSetHeight()".)
I think this should get some kind of resolution. Either feedback from the real experts whether there are fatal "gotchas" in deploying such a function, or that someone will actually write it if deemed a good thing. (If written, I'm not sure when it will actually be available in SL.)
We'll see. For reading my bounty offer, including an explanation for why I think this function would be useful, go to:
http://www.libsecondlife.org/wiki/Bounties