It is now possible to use only lidar with rtabmap without a camera. We should explicitly set subscribe_depth and subscribe_rgb to false.
The odometry can come from the robot, rtabmap_ros/icp_odometry node or another lidar-based odometry approach (like hectorslam). The setup would be similar to this config but without the camera (change this to name="subscribe_rgb" type="bool" value="false"): http://wiki.ros.org/rtabmap_ros/Tutorials/SetupOnYourRobot#Kinect_.2B-_2D_laser
The disadvantage of not using a camera is that visual loop closure detection will be disabled. Proximity detection based on lidar will still work, so small loop closures could be closed.