security is always a trade-off with convenience/usability and IMO that layer on top of plaintext protocol would be minimal comparing to already your OS surface. And if you go in that direction then why not go further? develop a basic custom minimalistic OS (in a way that compiled code could be verified in case of compiler backdoor) for just single purpose for secure messaging. It could be booted from CD-ROM or read-only flash, would self-verify itself and PC hardware for known anomalies, present you with a hash of environment so you've memorized it and if it ever changes you know someone have touched something on your PC, maybe BIOS, maybe other firmware maybe your boot medium etc. Then you would plugin your security token with encrypted GPG key and you could securely message. But actually no, you wouldn't use just general purpose computer, you would have developed a custom computer from ground-up with every single chip and transistor to be verifiable and it would serve only this single purpose of secure messaging. But now what if your friend doesn't do the same? it's all bets off and you've lost because it will be easier to "attach" to other end than you.
Anyway I see a reason for both of these use cases, encrypted feature full messaging and just extremely secure basic plaintext messaging. But if you go with latter then I wouldn't stop in middle that is I wouldn't use same general OS but something trimmed down. I think currently Tails is pretty good and it comes with
Pidgin OTR and you can use it over IRC network which is basically a simple plaintext protocol so your case is already covered I think.