That seems like the next thing. Make it work something like a death throes?
Sounds good! So, it imparts x negative levels on any creature of non-neutral alignment, and grants a wish to a neutral character that does not wish it harm?
If a silver dog is ever captured, whether by nets or spells, one of two things occurs. If the capturing character is any alignment but pure neutral, or if the character is of neutral alignment and means to harm the silver dog, the dog disintegrates, as per the wizard spell, and the character suffers a 1,000 XP loss. If the silver dog is captured by a character of neutral alignment who means the dog no harm, the character receives a wish and 1,000 XP. As soon as the wish is fulfilled, the silver dog disappears as per teleport without error. If ever a silver dog is cornered and slain, the slaying character suffers a loss of 1,000 XP.
Sounds good to me.
So maybe:
Silver Dog's Boon or Bane (Su): Capturing a silver dog is a risky proposition. A true neutral character that captures a silver dog and has no intent to harm, the silver dog will grant a wish (as the spell) and gains 2d4 positive levels that last for one week but never result in actual level gain. If a non-neutral character or one who whishes to harm the silver dog captures the creature, the silver dog disintegrates, and the character instead takes 2d4 negative levels. These never cause actual level loss, but they persist until the character receives a break enchantment (?) or similar spell. This same bane applies to any character who kills a silver dog.