Found this, will try it out.
https://protools.ideascale.com/a/dtd/External-midi-clock-to-Eleven-Rack/115780-3779Update: removing USB does allow MIDI to be selected under "tempo". In a quick test comparing echoes against a drum beat, it appears to work OK. When that happens, the delay time is shown and wiggles around a bit which hints that it's being calculated on the fly.