Hi,
I think there are some rosbridge to unity libraries out there (like this
one). Maybe there are some more recent. But if you find one that can convert PointCloud2 messages, you could get /rtabmap/cloud_map topic from rtabmap streamed to Unity. It won't be the most efficient (as using /rtabmap/mapData), but it can give an idea on how transferring data between ROS and Unity.
cheers,
Mathieu