Bitcoin is a digital currency that has to replace fiat money, so anniversary coin must also be digital, in my opinion, and every active valuable member of the forum should have the opportunity to get a collectible coin.
May as well go for individually tailored crypto kitties instead. That's if the forum can afford the ETH fees these days.
That looks like an extremely expensive and not all that meaningful option. It would be more profound to send everyone's UID in satoshis to the genesis block.