I'm really surprised that the dealership said they can't get you a replacement key. It's a GM branded accessory and the dealership should provide warranty/service for it. I might try calling a few other dealers just to see if you get the same story.
A few years back my family was given a Sears (Thule) roof cargo box. The keys were missing, but I found a locksmith on E-Bay that was selling pre-cut keys for cargo boxes. I found the number on the lock cylinder, ordered two keys and they work great.
Another option would be to contact Etrailer.com They have great customer service and deal with roof rails, cargo boxes, etc. Chances are those roof rails are made by someone like Thule or Yakima and then rebranded as GM. You might be able to get a replacement key through them...