Hi, I am also looking to do the same thing on 1130AGs. Going through all values in the dot11 association MIB including the one you mentioned doesn't appear to give any directly usable value.
You can get the vlan IDs for all clients using 220.127.116.11.18.104.22.168.222.214.171.124.1.17 (cDot11ClientVlanId), but then you would need to do some kind of operation on the results from that collection. If there is another easier way I'd be interested to know!
You can get the number of clients associated with each radio interface using 126.96.36.199.188.8.131.52184.108.40.206.1.1 (cDot11ActiveWirelessClients), instance 1 is the 'g' radio, and instance 2 is the 'a' radio on the 1130AG.