The only proof I have is the secret url which used to be enough evidence. But it's not anymore. Why? I don't know...
Times have changed and circumstances surrounding it have equally changed. The amount you claim to have say 1btc 5 years ago could be an amount someone can cough out before thinking twice but that is not the same now. So, at the time when the URL was sufficient, it was because Boussac could afford it without blinking but now when the service has been closed down, you will ask for additional evidence if you were in his shoes.