Publish a topic only locally?

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Is it possible to publish a topic only locally? I have a data structure from an external library that I'd like to make available via ROS to other subscribers in the same node (e.g. when using several nodelets in one node). However, this data structure is very expensive to serialize. So I'd like to make sure that external subscribers (most notably "rosbag record -a") cannot see the topic and cannot subscribe to it.