It is possible to use 2d arrays in the custom node but I was getting several issues with effectively looping them, and populating the array. The array dimensions had to be fixed for example. I will look more, maybe there is a way. I'm also not sure if it would be any cheaper then the manually unrolled loops. Its set up…
It still doesn't work in 4.24 but a fix has been published on this thread: https://forums.unrealengine.com/unreal-engine/feedback-for-epic/1618618-virtual-texturing-feedback?p=1688979#post1688979 However it implies recompiling the UE source code yourself. Hopefully the fix will make it into 4.24.1.