People on Disqus need to have a flattr account first, though. Preferable, people without an account would receive a message that people wanted to send them money for their comments, when people tried to flattr them.
It's easily doable - the only thing holding it back is Disqus's desire to innovate. So let's do our best to make sure it lands in Daniel Ha's desk.
It could also be interested to see it on some forums - but I'd worry that people might get an incentive to game the system. The same might apply to Disqus to some extent - go for the low-hanging fruit in a very partisan-lenient political discussion for instance. It might work on forums if it was only possible to flattr the opening posts and the work that went into making them. Gated forums where people are admitted at the admins' discretion might also make this work.