You can send an Object with SendMessage, and the object can be anything. I would advise not repurposing things like Vector3 for this, because that makes your code less understandable. (Even though it might be tempting to do that, take a minute and do it right instead; you will save much more than that later when you're not having to figure out what the heck x/y/z means in that context.)

class DamageInfo {
    var damage : int;
    var weapon : Weapon;
    var angle : float;
    function DamageInfo (damage : int, weapon : Weapon, angle : float) {
       this.damage = damage;
       this.weapon = weapon;
       this.angle = angle;
    }
}

...

SendMessageUpwards ("ApplyDamage", new DamageInfo(damage, weapon, angle), SendMessageOptions.DontRequireReceiver);

Well, yes, to a certain degree that is.

You could just send a Vector3, like this:

var damage : float;
var weapon : float;
var angle : float;

hit.collider.SendMessageUpwards("ReceiveMessage", Vector3(damage, weapon, angle), SendMessageOptions.DontRequireReceiver);

And you would receive the message like this:

function ReceiveMessage (info : Vector3){
    print("Damage = "+info.x+" Weapon = "+info.y+" Angle = "+info.z); 
}

And of course you access the data just like you would with a normal Vector3(using its x, y and z values).

Im not sure but i think you could send a List or an Array with SendMessage