I was't aware of this, however, if you are a P holder with a high number, you can be put on the Sig backup list, if anyone drops out, by just asking.
I know my chances are slim, as I am sure all the Sig holders will likely buy their cars, and I have no idea where I stand on this 'back-up' list. However, I figured it can't hurt being I am P#3748.
Either way, even if I don't get a Sig, I suspect my number will move up the list, as many P holders above me will opt to wait on the lower battery packs.
So, if you are likely me, and you didn't know there was a backup Sig list, all you have to is call your local dealer to be put on the list.