This is a great exploration, no doubt. Though, I’d think of this problem probably without an interface at all. As Golden Krishna writes in his book — The best interface is no interface, we need to embrace typical processes. What is a typical process while we want to get to our house say in the 100th floor — just walk into the elevator and reach our house with minimal effort. Now having to browse through a set of numbers (irrespective of how easy it is to navigate) is still an arduous task. How can this be done thinking of solutions without an interface? What if people using the elevator had some sort of an ID (RFID of sorts, maybe?) — the elevator manages to read the different IDs. Or maybe as people walk in they just swipe/scan their ID on a reader — which probably is the only interface on the elevator. Each ID is associated with a floor, and the elevator now knows what are the different floors it has to halt at. Let us assume guests use these elevators as well and they are provided with a temporary one time pass that could be scanned. This, I believe, would be a more efficient way of getting to 999th floor without having to actually type the number of select from a list for any type of user.
That being said, I really like your exploration and I look forward to more such design problems being tackled. Cheers!