Ok, came up with a plan. I'm using the scarface idea as a contingency in case convincing them fails, but I'm also dumping a lot of motes into the stunt to get them to trust us, so it's unlikely it'll end up being necessary.
[X] Plan Zero is Nobody
Preparations: You will have the Anuhles scout out a suitable abandoned building for a meeting with the terrorists in one of the ghettos accessible by the monorail. You'll also get a prepaid phone that you can call as a means to contact the terrorists. As part of a backup plan, you'll go to the meeting place early and use your Peripheral Essence to create a disguise above your actual costume - the costume will be identical to your actual Zero costume, but underneath the mask the individual will be a man whose face is scarred and burned such that his race and former identity couldn't be recognized. You'll do this well enough in advance that you'll have stopped glowing by the time the terrorists arrive. (should be an hour)
Guiding the terrorists to the meeing place: Have Milly deliver the phone to the Tokyo Tower lost and found so they can announce to Kallen Stadfelt to come pick it up, as in canon. She'll be in plain clothes watching from a distance and will text you when Kallen picks the phone up so you can give Kallen a call. From there you'll guide her and her group to your location.
The meeting: You will greet the group politely, and praise them for their performance in Shinjuku. After that, begin with a statement of what your goal is - the complete and total defeat of Britannia, and the liberation of all people who suffer under their rule, and that you want them to form the foundation of an army that will work towards that end. They will likely be incredulous about this goal, so in response you point out their own battle. Their group has no formal military training, and yet under your leadership they were able to inflict more casualties against Britannia in a single battle than any other Japanese group has since the miracle of Itsukushima. If it had not been for the white Knightmare, their victory would have been total, but even if they were forced to retreat Kallen still had the skill to hold her own and working together you both managed to damage it. Britannia may seem like an invincible behemoth, but that is nothing more than an illusion.
You'll then explain that even after their retreat, you continued working to further your goals to liberate Japan. Using some clandestine resources available to you, you had Clovis convinced that their stealing the poison gas capsule was actually a plot hatched by the Purebloods - hand them a newspaper with the article showing this if they haven't heard about it - by using the IFF data from the Knightmare you stole from a Pureblood as 'proof'. Because of this Clovis is going to tear his own government apart looking for 'traitors'. If everything goes as planned, the internal strife will make liberating Japan will become much easier. Even if you didn't win militarily, you still won a strategic victory. Given all you have told them, ask if they will follow you.
They will inevitably ask to see behind your mask. If they do so, explain that you have personal reasons for not revealing your identity for now, but you may reveal it to some of them in time. However, you also have ideological reasons - Zero is not a person, so much as a symbol for the oppressed to rally behind. (See stunt) CONTINGENCY: If they do not agree to follow you without showing what's behind the mask, you'll agree to show it to Kallen and Ohgi only on the (mundane) promise of secrecy, showing the scarred false face and giving them the implication that you do have a personal grudge against Britannia.
If they've agreed to follow your leadership, you'll have an ideological discussion about the methods you'll use. The important thing is that their group will need to give up terrorism. War against Britannia isn't just a military campaign, it's a war of ideas. Britannia uses every instance of terrorism to fuel their propaganda machine, turning the public against them. They use this to justify the oppression of the Numbers, claiming themselves to be more civilized and morally superior, dressing up their heinous actions in a veil of false righteousness. As such, those working under you must become a force for justice. They must not only win on the battlefield, but show that their cause is right. They will protect the innocent and punish the wicked, no matter their race. By doing so, they will expose Britannia for what it is and gather people to their cause.
CHARM USE: Spend six motes on Second Oramus Excellency to enhance your attempt to keep them from needing to see under your mask. Spend two motes enhancing your attack on Britannia's ideology/morality with First Ebon Dragon Excellency
STUNT: You knew they would ask to see behind your mask, but you are prepared for this. "I'm sorry, but I would prefer to remain anonymous." You hold up your hand as they begin to object. Your inner Essence flares inside you as you speak, twisting reality in your favor. "I admit, I have personal reasons for this, but there is another important reason. You must understand, Zero is not an individual. Zero is an idea, a symbol for the oppressed to rally behind. Zero is nothing, and nobody. And yet, at the same time he is every person who wishes for a better world, every individual oppressed by Britannia. Zero is enigmatic and unknowable, defying all odds to bring about the impossible. If the man behind the mask were to die, anyone could take his place if they have the will to never give up and fight against injustice. In time I may show you my face, but for now I ask that you trust me as you did in Shinjuku."
As far as Zero's costume goes...
[X] Let the GM decide.
Canon should definitely be the basis, but other than that I've got no clue. If we do end up with the canon costume I certainly won't complain.