I have not calibrated my camera and I don't believe @Genjou did either.
But you don't have to to use rtabmap, I made It, @Genjou and @Nico too.
The way we did it was using the map transform published on the /tf topic when the "SLAM" mode is active (when one of the topics like /pose,/quality or /keyframe is subscribed).
You can tell rtabmap not to use it's own visual odometry but to use odometry supplied by an external source. And you can tell it to listen to a particular tf frame instead of an odom topic.
With this setup you don't have to calibrate your cameras since you don't have to use the stereo odometry module.
By the way, you can compute the visual odometry using the RGBD info from the slamdunk cameras and that way you don't have to calibrate either.