Accumulation seems to not work outside of ice biomes when coupled with serene seasons. I'm guessing there's some sort of biome list that's being followed, instead of occurring anywhere it's actively snowing.

That makes sense. Would it be possible to add something for if it's snowing on a given block (and producing a single snow layer)?

All I do is check the biome temperature here:
https://github.com/cam72cam/WinterWonderLand/blob/master/src/main/java/cam72cam/winterwonderland/WinterWonderLand.java#L61