If you are using only the Aruba base OS and the SSID is utilizing 802.1x authentication, you can implement a user defined rule and apply it to the AAA profile of the VAP. It can be done either by assigning a VLAN or assigning a role that has a specific VLAN defined.
The DHCP App Note has covers this type of configuration: http://www.arubanetworks.com/pdf/technology/AOS-DHCP-FingerPrint-AppNote.pdf
An example configuration would be something similar....
aaa derivation-rules user iOS-Devices
set role condition dhcp-option equals "370103060F77FC" set-value iOS (assumes a role named iOS with a VLAN assigned)
OR
set vlan condition dhcp-option equals "370103060F77FC" set-value XXX (VLAN number)
aaa profile "existing-aaa-prof"
user-derivation-rules iOS-Devices