Is Raspbian an absolute requirement; as in "it won't work on OpenELEC or any other JeOS?
As of right now, yes. Raspbian Jessie or Jessie Lite is officially supported OS for Cayenne. We do have plans to port Cayenne to other OS, but it is farther back on the roadmap.
Let me know if you get it working on other OS?
I am willing to try. I have LibreELEC which is an even more modest version of OpenELEC. Failing that I am more than willing to buy another RPi3 and install Raspbian. Thanks