Whoops pressed enter accidentally...
So lets start with the BSB. Obviously he can't take a mundane shield as an option (pg 75 Empire).
It also says in the rulebook that a magic shield is treated as a normal shield. (pg 174 IoB rulebook)
To take a magic shield just seems like a violation of the spirit of the rules.
But whoever won on spirit!
On to the sword of Justice.
So a few people have suggested taking it on a wizard so that wounds caused by his spells can be re-rolled.
I imagine you might think i'll see this as a violation as well.
Well I don't actually.
The other magic weapons in the Empire book specifically refer to themselves when they state their magical qualities:
Hammer of Judgement: Models hit BY THIS HAMMER must take a toughness test for every hit suffered.
Dragon Bow: Any hits FROM THIS BOW count as being from a magical weapon.
and so on...
I had never considered this option until reading it here. Seems cheesy.