How can I have Postfix add a header to all outgoing mail based on command?

Question

I would want to have postfix to run the command: /usr/bin/hashcash -mXb 26 <RECIPIENT1> <RECIPIENT2> ... <RECIPIENTn>

and add the output of this command as a header. How can this be accomplished?

This command should be ran at the most final stage, best would be to process these commands at post-queue processing of outgoing mail, because when a mail is put in queue, then mail is correctly authenticated with regards to sender adress and sender IP, and when mail is in queue, it wont matter if computing takes 1 hour, no client or server will get a timeout.