Your PGP-aware mail agent should add a line to the text to be encrypted, consisting of a random number (hopefully very unguessable and fairly random) and an RFC822 header:
X-PGP-nonce: b1de70694f5f0824f89cb3f09aece01d
and replicate that in the RFC822 envelope. Put just the nonce value and not the header in the block to be encrypted if you're concerned about assisting a known-plaintext attack.
Actually, that doesn't work either -- if I wanted to forward the message you sent me to someone else to make them think that you sent it to them, I could just take the nonce and put that in the header of my forwarded message and it would match... No, you need to include the "to" and "cc" fields as well inside the signed message. But again, the MUA should do this, not PGP. -derek