I've just tried adding another mod's blocks to the minecraft:sand tag via a datapack, but aloe pips are still not plantable on it, leading me to believe the supported blocks are hardcoded.

This is actually quite strange; the Atmospheric code checks for Tags.Blocks.SAND_COLORLESS, which it includes its own arid sand in. However, when I do the same with a third party mod's sand block, it does not work...
Any insight would be appreciated, as I cannot tell if this is a bug or not.