That's my assumption as well. Take ~15 million BTC, deduct Satoshi's coins, lost coins, and all the coins people like me are sitting on (not in circulation) and I bet we're looking at a fraction of the total well under 50%. That strongly influences the conclusions that can be drawn.

Besides, they wouldn't need to move coins to prove ownership, right? Just sign messages with the keys holding the bulk of the coins? Wouldn't that be safer?