How about this:
Air in motion in the atmosphere will try to go the shortest way from high to low pressure. Thus it will follow a great circle track across the Earth. With the exception of the equator, all great circles have a North/South component. So air moving from e.g. 50N60W to 50N20W will not be moving due East, but will have a bit of East and a bit of North. Thus the argument that Coriolis "force" is a phenomenon resulting from air moving North or South over a rotating Earth (Coreolis being proportional to V Sin latitude) still applies.
(it might be bull**it, but it works for me)
BJ
PS, I hope in true instructor style you said something like, "excellent question, very well put. Lets have a coffee break and we'll continue this discussion later."