Aircraft do have vertical cg limitations, but they are generally a "design consideration" only, with the intent being that the eventual operators don't have to worry about vertical (and, indeed, also lateral) cg loading considerations, because the OEM has considered a representative cg envelope in all three axes (plus associated inertia effects as appropriate) in developing both the airframe and component loads, and assessing the aircraft handling characteristics
Thanks for addressing my question, rather than digressing into the question of vertical thrust/drag moments which is an issue obviously addressed at design stage. I'm still interested to know if (say) an A380 could be pushed outside of the flight control envelope by excessive loading on one deck. I suspect the answer is no, but am also interested to know if this has been modelled and tested?