I think the question is how to enable SFTP without allowing SSH. A while back, I was troubleshooting a godaddy site, and found that their support calls SFTP as FTP.
They provide SFTP without shell access.
The following entry in sshd_config converts ssh(openssh-server) into a sftp server. I suppose, this can be done in a read only authorized_keys too.
NOTE: Careful, this applies for all users. Don't get yourself locked out. Use another instance or some other daemon.