Despite the small additional load, IMHO, the best solution is going to be: 1) Use SBL+XBL, 2) scan the message contents during SMTP and reject messages above a relatively high threshold, 3) scan the message again during delivery so that users can have their individual preferences, and add any necessary headers in this phase.

You can't modify it, but the technique I use in pythonfilter is to modify the control files to mark all recipients complete, inject a new, modified message into the queue with the same control data, and then tell courier to accept the message. courierd will examine the message from SMTP and remove it because it's already done, and then deliver the modified message that the filter added to the queue.