I have a question. How will you be the counting posts? Have you written a script? I notice the signature is applied to a user's old posts too. I assume you'll count posts made after the forum member has 'signed up' in this thread. I'm just curious.
Another thing - are you sure you can afford this? Might you need to apply a cap somehow, or ask people to volunteer to do it for free?
What if NameNarwhal creates accounts called MetaexchangeSignatureNarwhal0, MetaexchangeSignatureNarwhal1, MetaexchangeSignatureNarwhal2, MetaexchangeSignatureNarwhal3...