I seem to recall once experimenting with placing the PGP signature into an email header field where it does not impinge on the content. Sadly I don't think there are standard or reliable ways of doing this that are guaranteed to be transmitted by every potential email transport (much less the archiving systems in which most mailing list content ends up). For the same reason, there are probably few email systems capable of producing or interpreting such signatures. Perhaps there are systems based on placing the body into an appropriate MIME envelope, but if so they can't be in very wide use.