The problem with making the session variable UserID in the PowerStore scenario is that it has complex logic on the checkout page so that it automatically registers and/or logs in a new user when they checkout... so by the time they hit the confirm page everyone has registered by filling out the checkout form.
If you wanted to treat the registration form differently than the checkout form then you would need to set an additional session variable on the login page... maybe RegUserID or something.
Then you would just create a discount rule:
Condition: Based on the existance of Session Variable
Amount: Based on Subtotal ( * 0.1 )