I think you can turn this (or any equivalent service like ssh-j.com) into an end-to-end encrypted option by relaying an SSH connection over which you send the data, instead of relaying the data.

Example:

  # receiver
  ssh top-secret@ssh-j.com -N -R ssh:22:localhost:22
  socat UNIX-RECV:/tmp/foobar - | my command

  # sender
  my-command | ssh -J top-secret@ssh-j.com ssh socat - UNIX-SENDTO:/tmp/foobar
  (log in with your creds to receiver host, enjoy end-to-end encryption)