Hi,
No. it is not possible to upgrade the firmware of an IAP through a controller coz the Controller image and the IAP ( VC) image are different.
Let me understand something here, why are you using a controller along with IAPs, IAP itself will have controller functionality. please clarify whether you are using IAPs or APs .
If you are using APs, AP always update it's code through the Controller.
Please feel free for any further query on this.