Here is what I have working but takes a long time to show results:

(let [session (bbssh/ssh "remote_host"
                         {:username "ubuntu"
                          :port 22
                          :identity "~/my_ssh_key"})]
  (-> (bbssh/exec session "ffmpeg -convert super long video" {:out :string})
      deref
      :out))

and here is the solution I'm moving towards:

(let [session (bbssh/ssh "remote_host"
                         {:username "ubuntu"
                          :port 22
                          :identity "~/my_ssh_key"})]
  (let [command (bbssh/exec session "ffmpeg -convert super long video" {:out :stream})]
    ;; (println (char(.read (:out command))) returns immediately here, but not print...
    ;; I was going to try to run some kind of long reduce to repeatly grab chars and
    ;; println them with every readline
    ))

actually I ended up getting it working using this... not sure if it's the most elegant solution but works ok so far

(let [session (bbssh/ssh "remote_host"
                         {:username "ubuntu"
                          :port 22
                          :identity "~/my_ssh_key"})]
  (let [command (bbssh/exec session "ffmpeg -convert super long video" {:out :stream})
        line (atom "")]
    (loop [c (.read (:out command))]
      (when (not= c -1)
        (if (= \newline (char c))
          (do
            (println @line)
            (reset! line ""))
          (swap! line  #(str % (char c))))
        (recur (.read (:out command)))))))

yeah this works. you can also use (with-open [stream (io/reader (:out command))] .. do something with .. (line-seq stream))