Correct me if I'm wrong.
What you really need is a cool looking world object to be used as an activator. Once the player enters you lock them into first person view only seeing the weapons attached to the world object. Lets say a mini gun on the right hand and a power fist on the Left. (Warhammer 40k Dreadknot, kind of thing.) Forcing the player to have hot key 1 for the power fist and, hot key 2 for mini gun.
Yea, you pretty much nailed it on the head. I shall clarify some things.
The armor is seen 3 times. The first time, is when the player walks into the Prometheus labs, after being instructed to prep the armor for combat. It is sitting on a podium in the middle of the room (made out of chunks of the Liberty Prime setup in the Citadel Lab) and it will be a static. It cant be interacted with at all. I can make it bigger by simply setting the scale for the reference in the geck. (Which as far as I have every known will make it the same size constantly) So it does not need to be scaled up in nifskope at all. It can be average power armor height, as to fit nicely onto the player. Then the player fixes it up, and its 'flown away' via vertibird. Then they go to where it will be used. Upon which they are forced to remove all inventory items and put on somelike like a 'pilots suit'. Then they activate it, and their screen fades to black, they hear some noises, and it fades in. I hope to have VATS disabled, if thats possible. And also just as you said, it would be fantastic to have like 2-3 weapons for them to use. Which will be hotkeyed automatically with FOSE. (At this point they are scaled up to like 2x normal size) They run around, do some fighting. At the end, screen fades to black, hear some robotic noises again, screen fades in. They are standing next to it, in a fixed spot. Again as a static object, and they are returned all their gear. And then they leave.
So yea, it needs to be a cool looking object, that will be a static in 2 places, and activator in 1. Honestly, they dont even have to be wearing it. I can just put on some BoS PA so long as you dont change the lower arms, and we make them have some kind of funky gloves on, and we hide the pipboy somehow, incase it can be seen. Also the weapons will probably require no ammo, so seeing the pipboy during reloading wont be an issue. (They cant roam the wasteland freely while wearing this suit. So while its incredibly powerful and awsome, its in a completely controlled environment, in my own custom worldspace)