Can you pick up a step in harness somewhere? I have some 10 lb fur babies, and I actually sew my own harnesses so I know that they are very secure.. but I prefer to use the step ins especially in cases where they get really excited and pull more.
If not, I think the button hole idea is a great one!! I don't ever trust d rings on stretchy material or even shirts that are loose.
And, btw we used to live right behind where our fireworks in town where done and the furbabies were outside the whole time and didn't mind it a bit, I'm sure your baby will do great!
