I don't know how it would work if the maker of a lower from an 80% died suddenly but there's no serial. I guess the estate would be the de-facto owner at that point and would probably be encouraged to have it serialized.
Well, most estates go to families, so for example, if (ok when) I kick the bucket, my boys will end up with a gun or two. Now, I don't own anything serial numberless at the moment, but if I did, my boys would become the owner of it. They would already have knowledge of it. Wouldn't be any different.
My mom put my name on everything she owns. She is still living. When she dies (which I hope is in 300 years) it all simply becomes mine and my sister's. There is no transfer of anything. Why would that be any different for a gun (non-NFA) with serial numbers or not? I doubt the ATF would show up at my wake, demanding to see any firearms that are changing hands. When my last one is of age, my wife and I will set up our stuff just like my mom did.
Who would encourage the owner? How would "they" ever know about an anonymous gun to begin with? It is not illegal to make a gun. Pretty sure it is not illegal to pass it on to your kid, spouse, etc.