Hi!
I'm a user of Netscape, an it doesn't sign any message using PGP. I was thinking about this and the idea I have was to code a kind of "pipe" between the client and the MTA, that filters the message, and sign the body of it using your PGP private key, and then relay it to your normal MTA.
I was to create a project in Sourceforge for it, but I find yours, and I have this...
2000-12-21 14:21:03 UTC in PGP Forwarding Server