Wouldn't using the mailing list's email address in the hashcash algorithm work? Then the mailing list's server or recipients' clients' spam filters could utilize the hashcash header, comparing it to the mailing list address in the To field.
Mailing lists could even demand a lower hash target, requiring more energy to send messages to lists. This could rate-limit flame wars, for example.
HashCash would make it more expensive for everybody to send email, not just spammers. It's also one of those things that doesn't really work unless everybody starts using it.
What are you going to do to emails without the HashCash header when 95% of the World is still sending email without it? Does it matter to anyone else what you do to those emails if you're the only one doing it?
HashCash would have some benefits, even if only a few people start using it, and would give incentives to participate: Just don't use it for a binary decision, but as one factor in your probabilistic spam filtering. And make the filter also discriminate on the amount of work put into the hash.
Spam filtering is already 99.x% effective. So you're basically increasing the cost for everyone in order to gain one extra SpamAssassin rule?
There are plenty of reasons why "Payment" anti-spam methods have been soundly debunked over the years. You're not the first one to think it's a good idea, and you probably won't be the last, but do the research yourself first into why they can't work.
Mailing lists could even demand a lower hash target, requiring more energy to send messages to lists. This could rate-limit flame wars, for example.