Ugo-Your email reminded me that I was thinking about adding this feature to joint_state_publisher. I've attached a version that I'll upload to our repository after a little more testing.
<node name="joint_state_publisher" type="joint_state_publisher" pkg="joint_state_publisher" output="screen">
<param name="robot_description" textfile="model.urdf" />
<rosparam param="source_list">[head_states, body_states]</rosparam>
</node>
This is how I launch it to merge the JointStates published on head_states and body_states.
Let me know how well it works.
-David!!