IMultiBlockHandler.java

public interface IMultiBlockHandler
{
    public void addMember(World world, int x, int y, int z);
    public void addSpecialHandler(Object obj);
}

Multiblock.java

public class BlockMyMultiblock extends Block
{
    public MyBlockMultiblock()
    {
        super(Material.rock);
    }

    public boolean isMultiBlock(World world, int x, int y, int z)
    {
        return true;
    }
    
    /* this data could be subject to change. Dont store it indefinitely. */
    public void populateMultiblock(World world, int x, int y, int z, IMultiBlockHandler handler)
    {
        handler.addMember(world, x, y, z); // adds this block
        handler.addSpecialHandler(world.getBlockTileEntity(x, y, z)); // adds in my TE for liquid handling.
    }
}

MultiBlockHandlerImpl.java

/** This is a dummy implementation. You would generally want to write your own. */
public class MultiBlockHandlerImpl implements IMultiBlockHandler
{
    /** DONT STORE THIS DATA PERMANENTLY */
    private final TIntHashMultimap<ChunkCoordinate> members = new TIntHashMultimap<ChunkCoordinate>();
    private final LinkedList<WeakReference<Object>> specials = new LinkedList<WeakReference<Object>>();

    public void addMember(World world, int x, int y, int z)
    {
         members.put(world.provider.dimensionId, new ChunkCoordinate(x, y, z));
    }

    public void addSpecialHandler(Object obj)
    {
        specials.add(new WeakReference(obj));
    }
}