So, imagine a unit of Grey Knights assaults/is assaulted by a unit of Thunder Wolf Cavalry with a Wolf Lord. The Lord has a Wolf Tail Talisman (dispels psychic powers that target him on a 5+). The Grey Knights pass their psychic test to 'activate' their force weapons.
Does the Wolf Lord get to try and dispel the power if he takes a wound?
Now, under the Brotherhood of Psykers, Grey Knight unit's force weapons work a little differently. The Grey Knights pass the test, and their weapons now inflict Instant Death. At no point, as far as I can tell, is the Wolf Lord actually targeted by the psychic power. The power just affects the Grey Knights. The Wolf Lord is hit by an attack that causes Instant Death, but that attack itself is not a psychic power.
As I see it, the Wolf Lord is screwed.
Here's another way of looking at it. Would a Wolf Lord get to dispel the Hammerhand Psychic power? Hammerhand targets the Grey Knights, and makes their attacks Str 5. The Wolf Lord takes a Str 5 hit, but is not actually targeted by a psychic power, so he gets no dispel. Now just replace "Str 5" with "Instant Death", and you have the force weapon activation.
Edit: Oh, and a second thing, just for confirmation.
A Grand Master is attached to another unit of Grey Knights. The GKs cast hammerhand, giving their unit +1 str. Because the Grand Master is part of the unit, he also benefits, gaining +1 str. While you allocate attacks as if the GM was a separate unit, the GM is still part of the unit of GKs for all other purposes. Since Hammerhand/Force Weapon activation isn't part of the attack allocation, you still treat the GM as part of the unit, so he still benefits.
Now, the GM does not have the Brotherhood of Psykers rule. That means that when he activates his Force Weapon, he does it normally (wait till after saves are made, then pick one unsaved wound and pass a psychic test to make it inflict Instant Death). However, if the unit of GKs activate their Force Weapons, the GM is included in the effect and all his attacks inflict Instant Death.
So, use the squad to activate Force Weapons (brotherhood banner anyone?), and use the GM to cast Hammerhand.