Thanks both for the suggestions, however:
Looking through the old code, the signature seems like it's the hex representation. Have you tried just converting it to base64?
Yes with
https://www.base64encode.org/ that's correct right? UTF-8 is also correct I assume?
Extract the private key from your address in Armory.
Import that private key into Core.
Sign whatever message you want with Core.
Be sure to retain your air gap in the process, if you currently have one.
It's a cold wallet so that's not acceptable. The private keys are never to hit a computer which touches (or has ever touched) the internet.