This would be relatively difficult to achieve, although clearly not impossible. For performance, the WQS API actually uses a hybrid approach to retrieve data from the repository: OpenCMIS and custom REST API. Therefore both of these interfaces would need to switch context together. Generally, I wouldn't recommend that you try to map your website visitors into Alfresco users on a one-to-one basis. Although Alfresco would cope with it, it's almost certainly putting far more stress on the system than you need to.
