Very interesting concept, but I had something much simpler (and less risky) in mind. User enters send/receive addresses in profile, and a button is displayed on every post (in the sig or sidebar or wherever) that allows anyone to micropay directly. The forum would not hold onto the BTC at all, just a direct payment. Or maybe you could skim off 1%, etc. You might need to ratchet up your servers' security a bit to prevent hackers from changing peoples' profile addresses, but it's a pretty simple plugin I'd imagine.
Bitcoin micropayments (tips) works on sites like Reddit, I believe, because of the visibility of the tips made against each comment or post. So as a community a new ethos is formed around tipping quality discussion and helpful/meaningful posts.
If as you suggest a button is displayed, then it also should provide visibility of the tips received. User preference should determine whether your forum name is displayed alongside the amount of the tips you make.
In terms of security, I propose that Hierarchical Deterministic Wallets described in
BIP0032 could provide the solution. Any forum member who wishes to receive tips creates their own HD Wallet and provides just the minimum chain code public key information to bitcointalk. This is enough for the forum to derive new child public keys without ever knowing the private key. To save on resources a child public key would be created just-in-time when the first tip is performed against a post or comment.
If what I've described is accurate then bitcoin micropayments could work on bitcointalk without becoming a target of hacking.