I'm not sure that's the only solution. What about breaking out the uid creation process away from the submitting of a completed order? Perhaps that should be the workflow-ng config. "Create user account at checkout" could be an action for the condition "User checks out anonymously".. something to that effect.

I think the main reason being you have the possibility of more modules in the future that might need the flexibility to act differently depending on if the order comes from an anonymous user or not. Specifying it just for Roles and File Downloads seems getting a little too specific to me. What do you think?