> is going to be short lived because someone will train an AI on a corpus of human keystrokes and get a replication

You can remotely attest the input devices. You can do it anonymously (long story, but doable) and without requiring some kind of pre-signed image for the while OS. (The OS passes through recent-input attestations.)

If it matters that much, and you still don't want to, just hire someone to type it in.