Why not just put tip addresses in signatures? There is one below. Feel free to try it out!
I already explained why - same reason as with the URI method.
The tip is supposed to be minimal, instant, from a specific forum user and for a specific forum post - only then it acts like the right incentive to improve quality of the content.
And it's supposed to be one-click only. Well, maybe two clicks - from any web browser, anywhere you are.
The problem is then that the forum must serve as a bank, a responsibility I am sure they do not want.
I like the idea, but implementation is unrealistic.
Bank - that's a big word.
It's not about big amounts. If you deposit i.e. 0.2 BTC it can last for weeks, even if you don't get back any tips yourself.
And the forum can outsource handling of the wallet/deposits to some other, trusted person or company - like instawallet, blockchain.info, mtgox, or whatever - satoshi's personal wallet
Everyone willing to deposit (or to not yet withdraw) would know where is his money and who is responsible for his tiny balance - the forum would have no liability here, only providing the user interface.
I don't think the total amount would be that huge to have a trust issue here - rather technical issues.
But if any forum in the world should be able to deal with the technical issues of implementing bitcoin micro-transfers, I would say bitcointalk.org is a good place to start.