A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
A player, 9 February 2012
So since again 'grants' the ability to use an ability a 2nd time (because normally it would be illegal) but it does not specifically give how its granting it.
Again ref:
Text Battle: Chose your performing Kensai: Straighten him and all his Weapons. Battle abilities on his Weapons may be used a second time this turn. You may take an additional Battle action.
So if the use of a weapons ability was denied by costly oppertunity, but the weapon had already been used once, would Again grant the use of the ability specifically for a second time? Costly opportunity obviously prevents the 1st use, and any beyond the 2nd in cases such as cursed relic. But Again specifically grants the ability to use it a second time and doesn't specify why you might not have been able to use it a second time before.
Costly Opportunity ref:
Battle: Destroy your performing unbowed Samurai and target an enemy card without attachments: Abilities on other cards now in its unit may not be used. Destroy the targeted card.
Jon Palmer (rules official), 9 February 2012
So since again 'grants' the ability to use an ability a 2nd time (because normally it would be illegal) but it does not specifically give how its granting it.
Again ref:
So if the use of a weapons ability was denied by costly oppertunity, but the weapon had already been used once, would Again grant the use of the ability specifically for a second time? Costly opportunity obviously prevents the 1st use, and any beyond the 2nd in cases such as cursed relic. But Again specifically grants the ability to use it a second time and doesn't specify why you might not have been able to use it a second time before.
Costly Opportunity ref:
No. The permission granted by Again! to allow the use of a second time will not override other effects that prevent all use of the item.