Please see page 181 on the document here: http://support.arubanetworks.com/DOCUMENTATION/tabid/77/DMXModule/512/Command/Core_Download/Default.aspx?EntryId=10987 for the workflow.
When a guest self-registers, enabling that option (Enable Guest login to a NAS) will put in another page where the guest can just click on a button and log him/herself in. If you have the option "Require sponsor confirmation prior to enabling the account", the guest must wait until the sponsor approves it, and the login page will reflect that".
You can simulate this by going through the guest self-registration. It only occurs when a guest self-registers.
Mac caching is the only other method that can automatically log a guest in, and you cannot redirect to a page where the username is put in automatically when the mac address is known.