You can edit this to anything already, even though the feature is NYI (Not Yet Implemented).
Example; Nothing but air, air everywhere! (aka blank map)
1. Make a new map
2. Select Superflat
3. Customize
4. Remove all layers except one, doesn't matter which one
5. Click presets
6. in the text field you might have something like "1;7;1", or "1;2;1", edit this to "1;0;1" (without the "" of course)
7. Tada! nothing but air!
8. But now you have a problem, you cant start to place any blocks anywhere
9. Use MCEdit and open the map
10. Select the brush tool
11. Find the spawnblock
12. Select "X:15, Y:1, Z:15" and place it near the spawn without clicking
13. Scroll with your mouse and adjust the circle till the center so it is right under the spawn.
14. Click so the circle is pasted into your map (center of circle should be right under spawn)
15. Click "Ctrl+S" to save your map
16. Click "Ctrl+W" to close your map
17. Exit MCEdit
18. Load your world, tada! now you have a 15 by 15 circle to spawn on in an empty world!
19. Build!
Not the best example really since I don't describe how to use the text field in the preset selection screen, but I haven't really had any use for it, so I haven't learned it yet. Study its structure and you'll see how it's built up.
