You might consider adding some custom code to the actual product list or detail pages that does something like:
------------
If (some session variable you get / create when the customer logs in exists) Then
price = regular price * rate
else
price = regular price
------------
If you do this, you can show the discount with the product information, and, you can also store the price (regular or discount) when the customer adds the item to their cart.
Doing this shows the discount in the individual item price in the cart display as opposed to as a discount below the subtotal.
This approach might be easier than trying to create discount rules in the cart behavior.
