I believe the use of a mirror should allow for establishing line of sight. However, line of effect is more the issue here. Most any attacking power requires line of effect to the target, or (for burst/blast effects) requires line of effect to an origin square from which there is line of effect to the target.
Especially, consider the verbiage noted in Magic Missile as you've cited:
The missile strikes unerringly, even if the target is in melee combat or has less than total cover or total concealment.
In most cases where you would want a mirror to establish line of sight, the target you're working against would have total cover from you. Since the mirror only helps you get around concealment, the total cover would render Magic Missile unusable.