FlatBedrock Xplosion's Edition

NBTTagCompound compound = new NBTTagCompound();
compound.setString("fileName", "testDimension");
compound.setInteger("dimensionID", 555);
compound.setBoolean("generateFlatTop", true);
compound.setBoolean("generateFlatBottom", false);
compound.setString("fillBlock", "minecraft:end_stone");
FMLInterModComms.sendRuntimeMessage(Reference.MOD_ID, Reference.MOD_ID, "addDimension", compound);

Recommended:
It's recommended that you install UniversalCore for extra utilities and functionality (explained below)

The mod on its own:
the mod on its own simply generates the bedrock in the overworld and nether (floor and roof) flat. Other dimensions can be added to that list by the player itself by creating a custom JSON file (see below at "add modded dimensions"). This should work for most of the modded dimensions that use the normal system of world generation. note: Mystcraft dimensions don't work for this but when it updates to 1.7.10 I'll add compatibility for them by adding a new Mystcraft page.

When using UniversalCore:
When you use UniversalCore together with FlatBedrock, FlatBedrock will get it's full blown VersionChecker which is also compatible with the VersionChecker mod. UniversalCore will also allow you to retroactively generate your Minecraft worlds (read more about this at "generating flat bedrock in an existing world"). I actually recommend using FlatBedrock with UniversalCore because this is such a small and more utility mod I figured it would be nice to not be dependent on other mods. you can get UniversalCore here.

Generating FlatBedrock in an existing world:
If this mod is installed alongside UniversalCore, this will add a way to regenerate the bedrock in your existing world to a nice flat layer. before doing make sure that you  Then you simply have to go to the dimensions folder, located at config/flatbedrock/dimensions/ and choose the the file that corresponds with the dimensions you want to regenerate and open it in a text editor of your choice (although I recommend using TextWrangler for mac and notepad++ for windows). after this you simple have to change either "retroGenBottom" or "retroGenTop" to true. When this is done you save the file and just reopen your beloved minecraft world but now it should have a flat bedrock layer.

Add modded dimensions (for users)
When a mod adds a custom dimensions and you to flatten it's bedrock you simply have to go to the config/flatbedrock/dimensions directory and create a new JSON file. The file has to be in the JSON format and should have following objects: "dimID" the dimension id, "genTop" wether the top of the dimension should be generated flat (like the nether roof), "genBottom" wether the bottom of the dimension should be generated flat, "retroGenTop" and "retroGenBottom" should only be used when UniversalCore is installed on you can find more about those above and the "fillBlock" a string value with the ID of the block that should be used for replacing the bedrock. you can look at the default overworld.json and nether.json files to look how it's used. Here is also an example of how it would be used by a modded dimension, twilightforest.json:

{
"dimID": 7,
"genTop": false,
"genBottom": true,
"retroGenTop": false,
"retroGenBottom": false,
"fillBlock": "minecraft:stone"
}

note: modders may have done this already in code so run minecraft first before creating a new JSON and look if it pops up.

Add modded dimension via IMC Messages (for modders):
modders can also send IMC Messages to register a dimension to the generation list like this.