Passwordless and keyless SSH guest access

What you're looking for is anonymous SSH access. I found an article for SFTP, which applies to SSH too, if you leave out SFTP-specific stuff:

1. Create a new user:

   adduser --disabled-password anonymous
   

2. Make the password actually empty:

   sed -i -re 's/^anonymous:[^:]+:/anonymous::/' /etc/passwd /etc/shadow
   

3. Allow blank passwords for SSH sessions in PAM: edit /etc/pam.d/sshd and replace the line that reads @include common-auth with:

   auth [success=1 default=ignore] pam_unix.so nullok
   auth requisite pam_deny.so
   auth required pam_permit.so
   

4. Allow blank passwords for SSH sessions of anonymous in /etc/ssh/sshd_config:

   Match user anonymous
       PermitEmptyPasswords yes
   

5. Restart sshd:

   initctl restart ssh
   

I didn't try it myself, but it looks plausible, since my first thought for the culprit was PAM.