Okay, I just completed the testing for the tractor transports, saves and results to follow.
Results
I placed my transport containing riots inside the threat ball on the Northern bordering world of Jatin in order to kidnap them for malicious experiments.
Once I deployed my Riots they immediately captured the enemy ships and returned to the transport. The ships that were captured would stay on the spot of tractoring. They would be able to fire on anything that got in range of them, but it seemed they were unable to move, as though in a constant state of being tractored by the riot ships, since they did not mobilize when I unloaded my merc forces on the same planet.
Upon unloading the riot ships I discovered that the enemy ships actually teleported to the riots and got recaptured by the tractor beams.
After saving the game, I quit to desktop and reloaded with the transport on top of my defenses. When I unloaded the transport the enemy ships teleported to my riots demonstrating that this would persist across saves.
Instructions for Recreation
The Transport that contains the Riots is group "5" and the riots themselves is group "2"
For Before, the transport is cloaked, and in position inside the minor threatball in Jatin. All that needs to be done is to unload and reload the transport of its riots. Then, you will be able to transport the enemy ships around at your fancy.
For the after, the transport is loaded with riot ships that have tractored enemy ships and is sitting on top of my defenses in the northern border world of Fiudon. you can see the ~500 ship group of Jatin turn into approx. 300 units once you unload the transport.
Conclusion
Currently, this could make riots and tractor platforms combined with assault transports a tedious and cheesy way to "encourage" large threat balls to attack.