You guys need to think outside the square a little here, this is easy to achieve when you put some thought into it.
Just have a webserver running some PHP or perl scripting and HTML pages to create accounts on the fly and import the account info into your usermanager box. You could automatically use the MAC address if you wanted, just grab it from the browser session. You could do this running usermanager on the hotspot or an external machine. We even have the option to SMS the username and password to new customers, it's really not that hard with some basic scripting knowledge and variables, and yes, you can do throttling, session management, and anything else you can do normally with hotspot user profiles and usermanager.