It's a bit more complicated. You'll need to modify CUO to support bigger Z, update uo net protocol packets and then update server scripts + packet handlers. After that you'll also have to update some tooling to support this If you choose reasonable values then it's doable. Same goes for map width and height.Wait, what?
I just never fiddled with z height because CED only allowed decorating between -127 and 128 so I figured the server had similar restrictions hard-coded. You mean, in theory, as long as you used ClassicUO you could throw a multi at z:10000 and have teleporter pads to get to it?