I'm trying to put together a team for tonight and started wondering about how ATAs interact with the reserve force when the Planet Hulk Dial is played:

BN Rulebook:
Planet Hulk Dial:
1. Since they are outside the game when the reserve is assembled, I assume this means they do not have the ATA assigned at this point?

2. Once they enter the game, do they then gain the ATA if they have the requisite keyword?

Event dials are outside the regular rules and, frankly, written with all the care of a rhinoceros giving you a back rub.

I'd say (and I think this was ruled this way regarding Peter Parker/Spider-Man AEs and the Outlaws ATA), you have to pay the points for any potential ATA users at the start, even if they aren't on your team at the time.

Are you saying that there was carelessness happening when they say things like roll 2 d6...on a result of 1, do X? (See Infinity Gauntlet)

Complete carelessness, in fact, more like half-buttedness. Wizkids released event dials and that's all they did. There have never been official rulings on how event dials work even though there was clear need of clarification and errata. I would contact your judge because he is your event dial authority for this event, what he says is how that event dial works for that event.

I would think that you treat the reinforcement team as a separate force. I would rule that you could use two different ATA's if you wanted to because in my mind, the reinforcement team is an entirely different force from the force played at the start of the game.

Regarding the Outlaws ATA you are correct. It all depends on if you want to be able to change Peter Parker into Spider-Man. If you are using the Outlaws ATA and you want Peter Parker to be able to change into Spider-Man then you must pay the points for the Outlaws ATA (because Spider-Man has the Outlaws keyword.) Because Peter Parker doesn't have the keyword, you could also not pay the points, but then Peter Parker would not be able to change into Spider-Man.

Who, me? Would I say such a thing? I don't know what you're talking about. : - )

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It was the last part I was unsure about. I guess the idea is that if you don't pay the points, then the Spider-Man Parker changed into would be an illegal figure, so that prevents the change. I'll add that to my list of internal justifications.