Tax Exempt Customers - Anonymous Checkout

I want to allow anonymous users/customers to:

1. self-identify as tax-exempt at checkout
2. supply a tax-exempt number at checkout
3. complete the purchase in the usual way but without taxes applied

I have http://drupal.org/project/uc_extra_fields_pane 6.x-1.0 installed to supply the necessary fields, but they don't seem to be accessible via conditional actions.

AFAIK, this can be done easily when the user is authenticated and can have a certain user role set. A conditional action could look up the customer's user role and exempt them from taxation.

How can I use conditional actions to exempt anonymous customers from taxes?

Thanks in advance!