WLAN Controller backup using Inventory Manager is working but you need to change some settings.
In IM go to the network elements tab and click on the WLAN Controller. Then on the right site click the Image Information tab and change Firmware Download MIB to Script and Device Family to Enterasys. (screenshot below).
Then in the option section of the IM set the SCP password - us the data of a Netsight Account. In my case I've put in the account information of the Netsight root user (it's a lab installation).