You're right about nihilitas, it seems to have shifted sense since classical times. I should have been double-checking my work with a medieval dictionary. I do like inanitas.
I agree that supernus is absolute rather than relative, but I read the English version as having the absolute meaning: "Only nothingness above [i.e., in the heavens, where you'd expect gods to be, but they aren't, so there's nothingness instead]" so it seems like it fits.
If anyone can do non-wrong Latin, I could use a translation of the following for HPMOR. The original is supposed to be circa 1200.
No rescuer hath the rescuer.
No Lord hath the champion,
no mother and no father,
only nothingness above.