Brought to you by ByteandaHalf & WorldOfGamingTV, the Flat World Layers mod allows you to customize the amount of layers and the blocks used in a flat world type. As shown by Herobrineskull, almost any block can be used with this mod and it is available for both Android and iOS devices.
Unfortunately, with an iOS device you must be jailbroken and have the following Cydia source added: mcpe.s1ris.com
As for Android users, feel free to download the mod from here: http://www.minecraftforum.net/forums/minecraft-pocket-edition/mcpe-mods-tools/2347759-mcpe-native-mod-superflat-world-layers-gui-edit
Don’t know any Block ID’s? We have got you covered: http://minecraft.gamepedia.com/Data_values_%28Pocket_Edition%29