I would require the player to qualify for the faction in game. Allowing to join KG without making the effert is simply too cheap and cheesy, considering that no other players are allowed a similar advantage.
You could make it an exercise or test that requires not using weapons or spells. So like, go perform a task in that manner sort of deal.
You could either roleplay it, or actually run a combat with the player needing to stick to unarmed combat.
It's really up to you.
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For an example, my brother just ran a session to have one of his players enter the Assassin Prestige Class.
The test he made him do was to race against another would-be Assassin to get a specific coin in a house a few streets over. How he got it didn't matter, but he was going to be watched the whole way.
He ran a chase scenario (like in the game mastery guide!) and when the two reached the door to the house, they had a scuffle. One even used dust of disappearance, thinking he'd for sure win (the NPC), while the player tossed out his dust of appearance, ha!
Eventually, after both were near death, the player Intimidated the other one into giving up and leaving.
The rest was roleplayed and he was successful in entering the Assassin guild (and got his first level of Assassin).
If you could set up a test similar to this, or a series of tests, this could be what the Faction needs for someone to enter, instead of a "promise not to use weapons for a month", which can be very detrimental and hobbling to the character.