"well, they didn't show me their individual tax documents nor did they publish their banking passwords!! SCAM!"
No one accuses AM of being a scam, mainly because they are more transparent and we can verify their status/hash rate.
At the beginning, AM went with pools, and it was very easy to see their hash rate. Additionally, we know AM's btc addresses and can follow mining revenue and hardware sales. We didn't need FC to reveal a lot of information and/or evidence, because we could verify that information first hand without him.
Maybe a signed message from the Labcoin mining address? That might be a simple option. Stamping the blocks would be good, too.