If I have a crafted item with a pattern from a reward applied, will this prevent the underlying item from being offered as ransom?
no. when you salvage something to get an eternal pattern, it loses ALL its attributes except for appearance, including any protections.
Actually, I am not really sure what you are saying. Let's say I have a crafted +13 sword. I apply an eternal pattern to it. Now is the sword itself protected from loss? In other words would the sword ever appear in the ransom window? On the other hand are you saying that the pattern has lost it's ability to be protected from loss because it is just a pattern and is now lootable like everything else?
yes, it would. when you salvaged something to get that eternal pattern, you gave up everything else that item had, including protection from ransom. you JUST get appearance.