Does Forge have a way to save a custom NBTTagCompound, which you can get when the world is reopened? If not, is there any way Minecraft Forge would be implementing such a feature?

Currently only possible if you are a coremod.

 

With a lot of coding, I worked out to have a cusotm world specific save file that can save and pull a sort of custom NBT kinda thing without being a coremod. It would be a lot better if Forge could just have you save custom NBTs in the world file...

Basically, I got the unique world specific file like this (for World--- the overworld outputs at ../saves/world---/data/AA/World0, For the Nether ../Data/AA/World-1, etc.):

 

String s = Minecraft.getMinecraftDir().getCanonicalPath() + "/saves/" + world.getSaveHandler().getSaveDirectoryName() + "/data/AA/World" + world.provider.dimensionId + ".dat";

 

Then I made a HashMap<string, Object>, similar to how NBT works. finaly I save the HashMap to the File using BufferedWriter and  FileOutputStream

There is a hook for this, but it's not all that straightforward to use. A World object on the server side has a field called perWorldStorage that holds a MapStorage instance. This is a keyed collection of WorldSavedData instances. You can add your own object to this collection and it will be saved along with the world.

 

Your object needs to extend WorldSavedData and provide readFromNBT and writeToNBT methods. An example would look something like this:

 

public class MyWorldData extends WorldSavedData {

final static String key = "my.unique.string";

// Fields containing your data here

public static MyWorldData forWorld(World world) {
                // Retrieves the MyWorldData instance for the given world, creating it if necessary
	MapStorage storage = world.perWorldStorage;
	MyWorldData result = (MyWorldData)storage.loadData(MyWorldData.class, key);
	if (result == null) {
		result = new MyWorldData();
		storage.setData(key, result);
	}
	return result;
}

@Override
public void readFromNBT(NBTTagCompound nbt) {
	// Get your data from the nbt here
}

@Override
public void writeToNBT(NBTTagCompound nbt) {
	// Put your data in the nbt here
}

}