Sunjammer has a library of custom functions for easily moving placeables around (or at least making placeables "appear" to be moving around).
If I recall correctly, it works much as 420 describes. It was scripted with NWN 2 in mind, but it is easily modified to be compatible with NWN 1. (One of the bioware functions he uses is only available in NWN 2, but you can replace it with little trouble)
You can find it on his site:
http://www.sunjammer.co.uk/Kind of curious about that memory leak myself.
Before I found SJ's placeable movement library of functions I had created my own elevator that visually moves up and down to those who are outside it. I did it with the destroy / create method on the 5 placeables that formed the structure of my elevator.
I've had no trouble with it and it at least "looks" quite good when it is in "motion", but I've not really put my module up online for any extended amount of time (yet) so don't know how well things will go in an active run of the module.