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I'm not sure why you want to do it with a topic. tf provides a very simple method to find transforms, the TransformListener. This gives you the ability to always look up the most recent transform between two frames, which is what I believe you are trying to do.

If, for some reason, you HAVE to subscribe to /tf, you can write a node that has a subscriber to the /tf topic, checks the "header.frame_id" and "child_frame_id" fields and determines whether or not to republish based on that information.