Okay I hadn't been checking beta recently to see that users had uploaded a custom mission with AI in it to the HTC server. Good to know though!
I am not sure, but I do believe that is so.... in fact, I had the same impression with the current live AH2. I think that feature may have been locked and only used by select individuals. If you look at the Aces High 2 Arena menu, there is one called "The Playroom". This was to run such missions that are now seen in the mission tab of beta.
Whether or not "Players" tab allows upload of mission type arenas, I would need to ask or test for myself. The overall impression still remains, that AI have been in use for some time now, and there has been no evidence that AH3 will place them in their main "Melee" tab.
If time allows I might attempt to load one and see if it works in beta with AI and specifically would like to try and make it WWI aircraft. The way the AI fly, that type of battle seems more suitable.