Hmm... Are objects such as signs also stored in the map model, or not?
If the map model is purely the terrain of the map, then I think it is achievable... A map editor could use a tiling system, which handles the x and y of each vertex, and automatically assigns textures based on the tile placed. Obviously, the tiling system would only be used in the editor, and converted to a model for the actual hack.
The z could be handled using a heightmap; compiling the map would simply take the x, y corresponding texture from a tilemap, and add the z values from a heightmap. The heightmap shouldn't be too hard to automatically generate from the way cliffs, ponds etc are laid out on the tilemap.
Buildings and trees are stored separately from the terrain model, right? In the editor, they'd be represented by large blocks of tiles, which can't be used separately, and placed on a separate layer. The bottom (terrain) layer would be compiled separately from the top (building and tree) layer.
At least, that's how I think it might be done... I'm not a hacker, so some of my assumptions are probably incorrect.