(Apologies if this is me being dense)
If you did find a way to deploy the key, surely you would break wireless connectivity for your users since you find yourself in a catch-22 situation.
Old WPA key is
abc (which the laptops already have stored). Your script deploys the new key
123. Wireless is now broken because the laptop now thinks the key is
123 but your access points know it is still
Conversely, you change your access points so the key is now
123, but your laptops still believe it to be
abc, thus they cannot connect to get group policy settings, to be deployed this new key.
You also have the problem, which has already been pointed out that the key can very easily be obtained. I personally have used such programs before as a favour to one of our employees who got a new laptop, didn't know their home wireless password but had it on their old laptop.