We've dealt with this type of question as well in our class. We also have a former boxer who is very fast with the left jab. After several unsuccessful attempts at applying kotegaeshi, hantai ikkyo, and kokyu nage, we discovered that he has a right arm!
Using that new found knowledge, instead of stepping off to our right (on the "safe" side), we did a choyaku movement, hooked his right elbow and turned him into a human slingshot several feet across the room. It took him completely off guard because his focus was on his left jab.
Another technique we used was to toss our sweat towels at him, giving us an opening to turn tail and run.