Aggravated Assault

Question: So, Brick Wall can't be used against aggravated attacks. Makes sense. However, if you use Divine Damage Conversion to reduce Agg to Lethal, could you then use Brick Wall to deal lethal damage?

No, you cannot. The boon says specifically that it cannot be used against attacks that deal aggravated damage, not just that it doesn't work on aggravated damage in general.

And really, it wouldn't make any sense anyway. Divine Damage Conversion is awesome, but it isn't really making the damage you just took not aggravated. Instead, it's you saying "I'm such a hardy, fortitudinous badass that I can deal with aggravated damage as if it were only lethal!" and acting accordingly; it represents the fact that you're so sturdy that even the most heinous damage can't hurt you the way it could hurt someone else. You take the damage as lethal because that's just how awesomely resistant to it you are, not because you somehow magically made the attack - which is under the control of your assailant - into a lethal attack. It's still an aggravated attack, but you just get to pretend it's not because of your adamantine hide.

Also, Brick Wall would be pretty busted if it could not only deal a bunch of damage to people who strike you, but also do so while negating all the resources that they spend on making that attack aggravated in the first place. So alas, you will have to continue only surprise-reflecting lethal and bashing into the faces of your disgruntled enemies.