Computing transforms in a Gazebo plugin

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I have a plugin, from which I'd like to modify some link parameters based on the position of the link relative to another link. What's the easiest way to get this information? My normal ROS instinct is to reach for tf2, but then I'm dependent on a bunch of external stuff which I really don't want to be, including robotstatepublisher.

So far, I'm just grabbing the GetWorldCoGPose for each link and subtracting. Is there a more elegant way to do this?