That is awesome. So to make this work, you need to replace the minecraft executable?Thanks! Yes, but I'm working on making my changes backwards-compatible, so that you can still use "normal" 256-block-tall maps in singleplayer and can still connect to regular 256-block-tall SMP servers. After I get that working, the next step will be to make this whatever sort of patch Minecraft mods use so that they can be combined with other mods, because I also want to be able to use one of the long-render-distance mods like Optifine to be able to see more of the generated skyline at once.
Edit: Thanks to tonight's coding work, my custom client can work with servers that are running both tall maps and normal maps, which means I (or you!) can play on regular servers with the modified client. The thing that doesn't yet exist is a server version that can understand tall maps and handle them properly; I'm not going to worry about that for now. I'll stick with loading my fledgling NYC chunks locally, then see if something like Bukkit or Spigot supports mods that go as deeply into the game core as my modifications do.