Lol
You don't have to look for an opportunity or even to bust a girls balls to attract her, because YOU are the opportunity. And besides, women don't have balls. That whole ball busting thing is cute, but its only an attack on the masculine front a lot of women are portraying today. So I guess its mens ways of putting them back into that feminine state. However if you simply just communicate with the feminine from the beginning, they will be just that.
Anyway, theres nothing wrong with you making a joke in a email as long as its funny. And I personally don't see the message you're thinking about as funny. Its just a bit try hard, even though its in good spirits. You are already rejecting her and thats not going to get you in. A message like:
" Lol I'm in. We can date.. How's tuesday ?

"
Would get you further than that; granted she's even sharing this possible fantasy of her taser partner being a romeo. Its your call man, just remember that she's no one to impress and she doesn't have balls. Draw women in and love them and you'll get a lot further. If you must bust balls, do so when they're getting out of hand.
Peace & Love
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