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You could use additional options from spawn_model (-z 0.5):

$ rosrun gazebo_ros spawn_model 
Commands:
    -[urdf|sdf|trimesh|gazebo] - specify incoming xml is urdf, sdf or trimesh format. gazebo arg is deprecated in ROS Hydro
    -[file|param|database] [<file_name>|<param_name>|<model_name>] - source of the model xml or the trimesh file
    -model <model_name> - name of the model to be spawned.
    -reference_frame <entity_name> - optinal: name of the model/body where initial pose is defined.
                                     If left empty or specified as "world", gazebo world frame is used.
    -gazebo_namespace <gazebo ros_namespace> - optional: ROS namespace of gazebo offered ROS interfaces.  Defaults to /gazebo/ (e.g. /gazebo/spawn_model).
    -robot_namespace <robot ros_namespace> - optional: change ROS namespace of gazebo-plugins.
    -unpause - optional: !!!Experimental!!! unpause physics after spawning model
    -wait - optional: !!!Experimental!!! wait for model to exist
    -trimesh_mass <mass in kg> - required if -trimesh is used: linear mass
    -trimesh_ixx <moment of inertia in kg*m^2> - required if -trimesh is used: moment of inertia about x-axis
    -trimesh_iyy <moment of inertia in kg*m^2> - required if -trimesh is used: moment of inertia about y-axis
    -trimesh_izz <moment of inertia in kg*m^2> - required if -trimesh is used: moment of inertia about z-axis
    -trimesh_gravity <bool> - required if -trimesh is used: gravity turned on for this trimesh model
    -trimesh_material <material name as a string> - required if -trimesh is used: E.g. Gazebo/Blue
    -trimesh_name <link name as a string> - required if -trimesh is used: name of the link containing the trimesh
    -x <x in meters> - optional: initial pose, use 0 if left out
    -y <y in meters> - optional: initial pose, use 0 if left out
    -z <z in meters> - optional: initial pose, use 0 if left out
    -R <roll in radians> - optional: initial pose, use 0 if left out
    -P <pitch in radians> - optional: initial pose, use 0 if left out
    -Y <yaw in radians> - optional: initial pose, use 0 if left out
    -J <joint_name joint_position> - optional: initialize the specified joint at the specified value

So something like this could work:

 <param name="robot_description" 
   command="$(find xacro)/xacro.py '$(find my_robot_description)/urdf/my_robot.xacro'"/>
<node name="urdf_spawner" pkg="gazebo_ros" type="spawn_model" respawn="false" output="screen"
  args="-z 0.5 -urdf -model my_robot -param robot_description"/>

On the other hand, I like the suggestion of supporting robot extensions that goes directly under the model tag.

You could use additional options from spawn_model (-z 0.5):

$ rosrun gazebo_ros spawn_model 
Commands:
    -[urdf|sdf|trimesh|gazebo] - specify incoming xml is urdf, sdf or trimesh format. gazebo arg is deprecated in ROS Hydro
    -[file|param|database] [<file_name>|<param_name>|<model_name>] - source of the model xml or the trimesh file
    -model <model_name> - name of the model to be spawned.
    -reference_frame <entity_name> - optinal: name of the model/body where initial pose is defined.
                                     If left empty or specified as "world", gazebo world frame is used.
    -gazebo_namespace <gazebo ros_namespace> - optional: ROS namespace of gazebo offered ROS interfaces.  Defaults to /gazebo/ (e.g. /gazebo/spawn_model).
    -robot_namespace <robot ros_namespace> - optional: change ROS namespace of gazebo-plugins.
    -unpause - optional: !!!Experimental!!! unpause physics after spawning model
    -wait - optional: !!!Experimental!!! wait for model to exist
    -trimesh_mass <mass in kg> - required if -trimesh is used: linear mass
    -trimesh_ixx <moment of inertia in kg*m^2> - required if -trimesh is used: moment of inertia about x-axis
    -trimesh_iyy <moment of inertia in kg*m^2> - required if -trimesh is used: moment of inertia about y-axis
    -trimesh_izz <moment of inertia in kg*m^2> - required if -trimesh is used: moment of inertia about z-axis
    -trimesh_gravity <bool> - required if -trimesh is used: gravity turned on for this trimesh model
    -trimesh_material <material name as a string> - required if -trimesh is used: E.g. Gazebo/Blue
    -trimesh_name <link name as a string> - required if -trimesh is used: name of the link containing the trimesh
    -x <x in meters> - optional: initial pose, use 0 if left out
    -y <y in meters> - optional: initial pose, use 0 if left out
    -z <z in meters> - optional: initial pose, use 0 if left out
    -R <roll in radians> - optional: initial pose, use 0 if left out
    -P <pitch in radians> - optional: initial pose, use 0 if left out
    -Y <yaw in radians> - optional: initial pose, use 0 if left out
    -J <joint_name joint_position> - optional: initialize the specified joint at the specified value

So something like this could work:

 <param name="robot_description" 
   command="$(find xacro)/xacro.py '$(find my_robot_description)/urdf/my_robot.xacro'"/>
<node name="urdf_spawner" pkg="gazebo_ros" type="spawn_model" respawn="false" output="screen"
  args="-z 0.5 -urdf -model my_robot -param robot_description"/>

On the other hand, I like the suggestion of supporting robot extensions that goes directly under the model tag. tag, issued