I have heard a few people asking about stuff like this, but the geographical reach of such a search would be really difficult.
My childhood ponies had my name on their hooves but some of them have since worn off, too.
And then there's the other problem of people who own ponies in their collections that are someone else's childhood pony. There's an ethical dilemma there that could be kicked up. It's a bit selective to only catalogue ponies that are not wanted by the person who finds them, and the chances are that we all own a lot of someone else's childhood ponies already.