Been a while since I tried Astral Sorcery out in depth but if this functionality is already implemented then you can get rid of this.

Any chance there can be a block that changes blockstate (to be detected by BUD's) or emits redstone when a certain constellation is present at night? Let's say you can configure the block by right clicking it with constellation paper, and when that constellation appears at night it will emit a redstone signal. In my developing modpack I'm thinking of using Horologium Attuned Crystal as a part of progression (since to my understanding its the least frequent appearing constellation) but I don't want to make it frustratingly difficult for players on multiplayer servers. As far as I know I don't know of any detection system present in Astral to help with this.

I'm hoping this will make it quite a challenge if a player wanted to automate getting crystals of certain attunements while not really changing a lot of the aesthetics of the mod.

Horologium only appears after an eclipse, so that one isn't particularly difficult to find.

Might be neat for automation, though.

Yea, you can. Build the relays in the specific positions around the attunement altar and if the constellation is active, you get a visual indicator that you should already be familiar with. :P