How to add a ground plane to MoveIt scene?

edit

The easiest way would be to add a box underneath the robot in the URDF that your moveit config is based on, as described in this tutorial. You could add something like this to your URDF:

<link name="ground_plane_box">
    <visual>
        <origin xyz="0 0 0" rpy="0 0 0"/>
        <geometry>
            <box size="1.0 1.0 0.5"/> 
        </geometry>
        <material name="Grey">
            <color rgba="0.7 0.7 0.7 1.0"/>
        </material>
    </visual>
    <collision>
        <origin xyz="0 0 0" rpy="0 0 0"/>
        <geometry>
            <box size="1.0 1.0 0.5"/> 
        </geometry>
    </collision>
</link>

<joint name="ground_plane_box" type="fixed">
    <parent link="world" />
    <child link="ground_plane_box" />
    <origin xyz="0 0 -0.25" rpy="0 0 0" />
</joint>

edit:

I didn't finish reading your post. You want to specifically add the plane, not a box as a workaround (although an advantage of using a box as a robot base is that it will be there immediately when you start the robot, and does not have to be initialized in code).

Anyway, your code looks good to me. Have you tried running it? There may be no visualization of the plane in Rviz, but if you drag a robot via an interactive marker into the ground, you could see if collisions are detected.