[exim] Add x-header based on subject?

I have a need to add "X-Sophos-SPX-Encrypt: yes" to the header on outbound messages, but only if the message contains a certain string anywhere in the subject line such as "spxencrypt". In addition to adding the header, I want to have "spxencrypt" stripped off the subject line as well. Is this possible with exim? How?

Thanks,
Alex