Hey Matt,
I haven’t dug far enough into the code to see where it’s actually prohibited (since SetInfo/GetInfo is on base SdfSpec), but if it doesn’t actually parse, I suspect the reasoning is that, were we to allow it, we’d really actually need two V’s in LIVVRPS, for metadata, at least, and rationalize why VariantSet is stronger than Variant, or vice-versa.
Relatedly, just to clarify, you can author metadata to Variants themselves… there’s just no USD-level API for retrieving it (guessing you were thinking about on UsdVariantSet?), other than inspecting UsdPrimStacks or UsdPrimCompositionQuery’s.