Isn't the "dead simple solution":
1. Write the message.
2. Encrypt the message.
3. Paste the encrypted message into the email client.
Your odds of accidentally sending a message in the clear are zero.
1. Write the message.
2. Encrypt the message.
3. Paste the encrypted message into the email client.
Your odds of accidentally sending a message in the clear are zero.
As the number of people on the email chain approaches 2, the chance of someone accidentally copying+pasting the entire email chain decrypted into a reply reaches shockingly high levels.
A painful manual process where the simpler slightly-less painful path works, but results in insecurity happening, are not a good way to make sure security happens.
Heck, people (myself included) regularly mess up Reply and Reply All :-)