Putting a jar inside a pressure cooker is no more dangerous that using a pressure cooker in the first place. But, it will keep the tobacco from just getting drenched in the water that you are using to build up pressure. If you are hesitant at all in using a pressure cooker, you may want to reconsider this whole line of thought.
I find using a pressure cooker to be fairly elementary. My daughters have been helping us can food since they were four years old. But, I also have no issue with menial things like fueling up my own internal combustion machines. But, there seems to be idiots who harm themselves doing this simple stuff; thus, we have over engineered gas cans hitting the markets. I don't know you, so I can't recommend either way. Just do what you feel comfortable doing.