Add Tax Exempt as an Option in Coupon Admin

I posted this query in another forum but I think this option is much better/easier than what we originally planned to do.

I'm putting together a site for an old mail order and phone order company. They have a huge customer base with customer numbers. Now that they want to focus on the website we will need for customers to enter their customer number and this customer number should trigger one or all of the following possibilities:

1. Tax Exempt
2. Free Shipping
3. Free Shipping after Set Amount
4. Discount after set amount

The last three are things already found in the coupon admin but how difficult would it be to add tax exempt as an option?

I also am aware that free shipping and amount discounts cannot be combined. Is there a reason for this? Will it break the cart if this is done? I'm just not sure why this would not be a great option for coupons.

Once this functionality is done we would create a coupon code for every customer based on their customer #. The customer would then enter their customer # at checkout in the coupon box which is now renamed Customer # instead of Discount Coupon.

Does all this sound doable?

Any help or point in the right direction would be AWESOME!

Thanks in advance!