What you could possibly do is.
1. Add a coupon with 10% discount.
2. Set the Uses per coupon to "Blank" with no alphabets or numbers in it.
3. Set the Uses per customer to "1" (This allows one customer, to use this coupon only once)
4. Set a registration email, which is when a user registers for the first time, send him an email (with the predefined coupon code) and highlight the same in the email.
Or you could even, on sucessfull registration, on the next page / or even on the coupon code entry module add the text with the predefined coupon code.
Note : This is just a solution to your question, however you could raise another post possibly asking for such an extension/module to be incorporated in the next release (to the site admins). if this does not exist now.