Just let them have the weapon. It's not like the choices are precisely balanced. If that causes everyone to throw out alignment weapons in pursuit of just the right one they want............is that really so bad?
I support this 100%.
Ignoring rules for the sake of cool and fun is always the best way to do things in my opinion. If the player likes the idea, and you like the idea, just say he's good to go.
If you really wanted to formalise it, I'd make a new Alignment that is a duplicate of the original, but with a different name and weapon - instead of Alignment: The Order of Orobas
he might have Alignment: The Knightly Order of Orobas
, which functions as their martial wing, rather then their spiritual side [and has a suitable weapon].
EDIT: In essence, what Krensky said...