When putting your opponent into the position for a rear naked choke it is common practice to open up the submission by bending the opponents spine backwards. Is there any realistic possibility of injury to the spine from this technique?
Yes, it is common to bend the spine backwards. It's called "hipping in." You pull their lower body down with your legs (your "hooks"), and pull their neck backward, forcing them to bend backward. This opens the person's neck up even more, and makes them extremely uncomfortable, increasing the likelihood of getting the tap.
A couple bonuses to make your RNC extra deadly: go belly down. There's no escaping a belly down RNC one you hip in! And of course, wear a cup. Few things feel worse than a cup digging in to your spine while you're already being choked to death.
As for a spinal injury, I've never heard of spine injuries from a RNC, but anything is possible! If you're just training in the gym and something doesn't feel right, tap!
If you are uncomfortable with something that is happening to you, then tap. If your partner taps, then stop and let go with no questions asked. All questions wait until after disengagement. This is basic safety.
Crazy things happen. I have seen something done safely 10,000 times, but also one time that resulted in a significant injury. If pain is telling you that the low probability event may be occurring, then tap and stop.
Do NOT mess around with spine injuries. These can result in permanent paralysis.
I would guess if there is uncomfortable pressure on your spine from a rear naked choke, then you are face down. In this position, some of your vertebrae may be restricted in movement, and force can be concentrated a few joints.