That is fair - there are almost no other subspecies of leopard in the USA. However, Maryland zoo holds Africans, and so does Zoo of Acadiana.
As for the AZA support, I believe that if they explained that they really wanted leopards and there are certainly other species than the amur leopard that need helping, they would be willing to make concessions and allow the import of North Persian leopards or Sri lankan leopards.
Considering the aza population of Amur leopard is just 120 cats, I do not believe another subspecies is a good idea. First of all it may eat into the amur leopard population, and second of all any secondary population of leopard would not be sustainable without a massive increase in new exhibit space (if it were to not eat into other species spaces). Also it’s important to take into account it has taken the ssp many years to phase out generic leopards just to attain a sustainable amur leopard population.