This sounds like the same problem that uc_roles had with giving permission to anonymous purchasers. The trick is to make the order go to the completed status after the user account has been created, not when the payment is complete. The Workflow event should be "customer completes checkout", the condition "check the order balance ($0)", and the action "update the order status (completed)". With roles, I noticed a slight delay in the processing, so that on the next page the customer saw, they didn't have permissions until the next page load. But if they have to log in, that wouldn't be a problem.