Instead of a tool that populates ~/.ssh/authorized_keys for all users, we could have sshd look up authorized keys directly from an LDAP tree (or whatever) using AuthorizedKeyCommand. FreeIPA sets up this with a sssd shim called /usr/bin/sss_ssh_authorizedkeys. The problem with passwdaliases remain, though.
sssd supports quite a few backends now, so perhaps we don't need to do our own tool, but rather work well in a sssd environment?