This is an awesome idea that we just implemented and that caused another large improvement in the amount of spam that is successfully detected and dropped by the mail servers. Sane Security provides virus, phishing and scam signatures for ClamAV that can detect most known mails with such content. Quite clever and effective.
Using a virus scanner to check for known content signatures is much more efficient than pushing an email through a bunch of algorithms to analyze individual words of an email. Both Bayesian filtering and Spam Assassin are generally quite CPU-intensive so this should give your system a break.
More info here.