For MMA training, elbows to the "back" of an opponent would only leave the spine and kidneys as viable targets. I know for sure that attacks to the spine, base of the spine/back of the head are definitely illegal so it isn't a readily viable tactic in an MMA fight. Also, as noted above, it would be better to work on setting underhooks when sprawling first. Not only is it a base detail that makes a world of difference to TD defense, it leaves you in a safer position if you can't readily put a full stop to a TD. At the very least you will end up in a neutral position.
For self defense purposes, it is very easy to train dropping an elbow when you sprawl. The same goes for kneeing the back of someone's head when they are fully caught under you if sprawl well enough. In the clip below, Jimmy Smith (pro MMA fighter) of the show "Fight Quest" is sparring with some Kajukenbo practitioners. He gets stopped by the last guy he goes up against do to an elbow to the spine. Granted the guy has a lot of height and weight on him, but it goes to prove that it is a viable self defense technique.
The vid is here.
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