I think what John is writing is that changes would have to be made to Dovecot in order for it to work.
I'm not going to make changes to Dovecot that's going to require it be repatched, checked, and reocmpiled every time Dovecot is updated. Do you want to take responsibility for being the Dovecot maintainer for DA

?
Seriously, it seems like a lot of responsibility to take on for something that would deliver the spam to the client's spambox, when the user who wants to read spam from a POP3 client can simply let SpamAssassin deliver it to the inbox and then filter it locally (using the desktop mail client) based on the (optionally) changed subject line.
Jeff