Would AI show as player #s on the main menu arena list?
No, but it gives people a reason to spend time in there.
In WWI, for example, there are people who enjoy that genre. But they don't go in there because there is no one in there. And no one is in there , because no one goes in there.
If someone went in there, there is nothing to do but fly in circles for a while waiting. They leave, and then someone looks and there is no one in there, so they don't go in there.
AI augmented arenas would prime the pump. People might go in there to futz with AI for a while. Someone comes by and sees there is someone in there, so they go in. Pretty soon, you have a ball-game.