weapon for a bodyguard

[King of Nowhere]

i have built this npc bodyguard focused on protecting his allies. he goes full combat expertise and shares the ac bonus with adjacent allies, he also shares the bonus for fighting defensively with allies, he can parry an attack against an ally with a reflex save, he can swap place with an ally that's being attacked, he can share several boosts to saving throws.
relevant here, though, is that he can make an attack of opportunity against someone attacking an ally, and he can give allies a flanking bonus even when adjacent to them.

but what should he do with his attack?

he's going to be taking a permanent -9 to hit (likely to become a -11 for use of constant guardian). he has a shield, so he can't do much damage anyway. the aid another action would be great, if it was a single attack; with 3 iterative attack this guy could give a +6 to hit. but it's a standard action. can't be used as an attack of opportunity. and he doesn't have the strenght to trip or disarm.

is there some weapon with some magical property that would make his attacks meaningful?

[H_H_F_F]

i have built this npc bodyguard focused on protecting his allies. he goes full combat expertise and shares the ac bonus with adjacent allies, he also shares the bonus for fighting defensively with allies, he can parry an attack against an ally with a reflex save, he can swap place with an ally that's being attacked, he can share several boosts to saving throws.
relevant here, though, is that he can make an attack of opportunity against someone attacking an ally, and he can give allies a flanking bonus even when adjacent to them.

but what should he do with his attack?

he's going to be taking a permanent -9 to hit (likely to become a -11 for use of constant guardian). he has a shield, so he can't do much damage anyway. the aid another action would be great, if it was a single attack; with 3 iterative attack this guy could give a +6 to hit. but it's a standard action. can't be used as an attack of opportunity. and he doesn't have the strenght to trip or disarm.

is there some weapon with some magical property that would make his attacks meaningful?

This is by no means optimal, but it fits the chassis: Have you considered getting the hindering opportunist feat from PHB2? It allows you to replace AOO with aid another.

[darkdragoon]

A martial Discipline weapon (Iron Heart)+ Stormguard Warrior feat might be able to offset most of it?

[Doctor Despair]

Tripping is always an option. Touch attacks are a lot easier than breaking regular AC anyway

[Gruftzwerg]

Maybe do some non-attack actions?
e.g. draw & drop Marbles into Enemies square

[Troacctid]

Broadblade short sword (CAd) gives you extra AC when fighting defensively, but it is an exotic weapon. I think what you really want, though, is reach. If we're talking martial weapons, maybe a ranseur.

[Darrin]

Poison Ring (Dragon Compendium): It's a simple weapon, only does 1 damage, but you make touch attacks with it, and you can load it up with some poison or a debuff. Aboleth mucus is cheap (20 GP), DC 19 or the target suffocates to death. Or use drow sleep poison for save vs. sleep. Roach Paste (50 GP, DotU) is pretty cheap for save vs. nausea. Multiple applications of blister oil can be stacked on the ring for a blister oil bomb.

Net (Core) or Lasso (BoED) are exotic, but they can be thrown as a touch attack. Or you could throw tanglefoot bags, which are not exotic, but have a similar effect. You may need Quick Draw to to keep up a decent rate of fire.

Sprayer (A&EG): This is not a weapon, exactly, but using it is a standard action, and you can "attack" with it without making an attack roll. You can fill it with an alchemical liquid, poison, aboleth mucus, etc. It creates a 5'x10' cloud that automatically doses anyone in the cloud with the liquid.

Sand Pipe (Secrets of Xendrik): This is an exotic weapon, but since you don't make an attack roll with it, profiency or attack penalties don't matter. As a standard action, you can blow the sand pipe to produce a 15' cone, so it covers more squares than the sprayer. There are three types of sand: blinding (Ref save vs. blindness), burning (1d6 fire damage, Ref save for half), or scalding (meh, skip it).

Guisarme or flail (Core): As Doctor Despair suggests, tripping is a pretty effective debuff. Guisarme has reach, which can discourage enemies from moving through threatened squares. Sometimes just having the threat there is enough to give you a small tactical advantage. If you can't spare two hands for a pole-arm, you can try the kusari-gama (DMG) for an exotic one-handed tripping weapon with reach, or just use a flail.