There are free versions of PGP for pretty much every platform - see http://www.pgpi.org/products/pgp/versions/freeware/ for details. But having said that, many corporate desktop computers are locked down to such an extent that users can't install any third-party software, and in most cases I guess the system administrators wouldn't install it just because one of their users wants to post to a mailing list. So that (plus the processing power issue) effectively rules out requiring PGP to authenticate postings. We're left with just the "include a password/unique code in the message body" idea, unless anyone has any more bright ideas on how to solve this.

Edit: chrislepingwell beat me to it, sorry if this sounds like a duplicate post :)

Edit 2: the freeware version of PGP more than likely wouldn't be usable by most people anyway, according to the licensing details at http://www.pgp.com/products/freeware.html