I figure I will follow up on this situation. I saw elsewhere that CZ now 'claims' they have on-line seat assignments for all tickets purchased on their web site. I am not yet convinced that they have a web site that is actually useful for anything but that is what they say.

But in any case, since I have a CZ flight coming up I figured I would call the Los Angeles reservation office and give it another shot at getting advance seat assignments. To my surprise, the nice lady reservation agent handled the whole phone call with dazzling efficiency and very quickly was able to assign me my choice of seats on all of my flights, even when it was on a CO ticket and reservation. We will see if the seat assignments actually materialize on flight day but I was pleasantly surprised by how nice easy it was to deal with the LA office this time around.