All,
I had a customer where I was trying Clearpass to authenticate Guest user against the fortigate firewall built in Wireless Controller.
I used the following URL to setup the fortigate and to authenticate the user http://cookbook.fortinet.com/using-an-external-captive-portal-for-wifi-security/
I was able to work with the Aruba TAC Rajesh and we were able to achieve full authentication by having the custom attribute under the NAS vendor setting of clearpass.
Please see screenshot... for reference.