With one of my upstreams (a large Tier 1 you've heard of), I can announce any prefix I want. I just have to add a "route" entry to the routing registry database

With one of my upstreams (a large Tier 1 you've heard of), I can announce any prefix I want. I just have to add a "route" entry to the routing registry database and wait a day or two for them to update their filters on my BGP session. It's easily doable (I do it occasionally so I can announce customers' prefixes for them) but I'd "get caught" if I was announcing prefixes I shouldn't be. BGP on the public Internet is all about trust.