I did also try setting "up" to be "down" with this hack:-
Vector3f.UNIT_Y.y = -1;
cam.lookAt([...], new Vector3f(0, -1, 0));
..but this didn't seem to change anything.
Does anyone know if this is a known issue with JME/Cardboard or is it just me? And/or are there any open-source JME projects that anyone knows about that might have solved this problem? Thanks!