How to associate only one public key with a restricted shell like scponly?

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http://unix.stackexchange.com – I'd like to use a passwordless key to perform e.g. unison synchronization while being able to SSH into the server only with a password-protected key. The usual way of using scponly is changig the login-shell of my server account, but that is too global. Can an entry in authorized_keys achieve this instead? (HowTos)