We are developing a solution that uses CMX to track people inside a specific building. But, when a user is connected to the Access Point nothing is shown on "UserName" field.
In our solution, we could associate a MacAddress to a single person - but, there is a problem! iPhone/MAC users! Each time they connect to the network, their macAddress changes! So we cannot trust this solution!
When connected to an AP, with dot11Status of Associated, the iOS device is using the real MAC address; while passively scanning, the MAC address is randomized in iOS 8 and later. However, the device can spoof the MAC address (with app) to connect anonymously.
Regarding using the UserName field in CMX, please see "The CMX Connect & Engage Service" in Cisco Connected Mobile Experiences Configuration Guide, Release 10.2 - Cisco
Also, see Wireless LAN Controller Web Authentication Configuration Example - Cisco for information on 802.1x username on WLC. There is also a nice blog on WLC - Web authentication here https://mrncciew.com/2013/03/21/wlc-web-authentication/