I've just found something about prespective distortion of circles inte ellipses. It has to do with two concentric circles, but the concept might be useful in my problem with only one circle, if advantage can be taken from the information about points A and B to create, for example, two chords through A and B respectively, and relate the points where those chords cross the North-South-diameter line???
Please help me THINK!
I here cite the (most) relevant section (3.2) out of a paper by Vincent Fremont and Ryad Chellali of Ecole des Mines de Nantes: "Direct Camera Calibration using Two Concentric Circles from a Single View" available at: link to paper.
At least my intuition that: "the center of projected concentric circles always lies on a line definied by the ellipses centers under ANY projective transformation"