First, you will need to check with the manufacturer to make sure that their vessel will handle the stresses. You could also have a good mechanical engineer take a look at it.
We put redundant vacuum relief valves on ours and we have a plumbing design that allows all of the steam to condense before the cold water is added. If you add the cold water into the jacket with steam in the jacket there will be very violent pressure waves that could damage or destroy your vessel. I have a plumbing design and operating instructions that we use for our pro series mash tuns, that are capable of steam heat and cooling in the same jacket.
However, I decided a couple of weeks ago that I will only be giving our steam/cooling jacket plumbing design to people that are purchasing our pro series mash tuns. I just don't want the liability.
You asked this question: "Our concern is the dramatic temperature drop from about 200F to 50F water. How would I introduce the water without doing damage to my jacket?" Answer this question. Do you do any damage to your vessel when you have 53 F water in your pot and you put 230F steam in the jacket? What if you are using a jacket to crash cool 200F mash and you have 28F glycol in the jacket?
What about a 304 stainless HLT with a 304 stainless fire box with jet burner flames at 1200 F on one side and 55 F water on the other side with a stainless vent pipe that is 600 F,and on my HLTs, I run the vent pipe up through the water. Here with this last example we have temp differentials as high as 1,150 F.
Always step outside of the box so that you can walk around the problem and look at it in your mind from all angles. Use common sense and logic.
I will tell you that if I had a choice between cooling with a steam, jacket or cooling with a tube in tube heat exchanger, I would go with the heat exchanger. Our largest model, like the one in the pictures that are posted earlier in this thread are really heavy duty, and our price is over $1,000 less than our closest competitor.