Hi,
No, it doesn't publish /map -> /base_link as a pose or odometry message. However, to see map graph's poses, you can add the rtabmap_ros/MapGraph display to RVIZ, and make it subscribe to /rtabmap/mapGraph topic.
If you want to republish an Odometry or Pose message, you can subscribe to /rtabmap/mapGraph, and republish the last pose in "poses" vector (see
rtabmap_ros/MapGraph msg definition). The "poses" are the poses of /base_link according to /map frame.
cheers,
Mathieu