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I don't know if this is the best approach, but it should work. I would write a node that publishes the transformations between /map and /base_link. In order to get the transformation you should use "lookupTransform" as you can see here:

http://wiki.ros.org/tf/Tutorials/Writing%20a%20tf%20listener%20%28C%2B%2B%29

As you can see, the transformation can be published to a topic of your choice. Whatever it is, you can then use this command in order to record it:

rosbag record -O subset /chosentopic