I don't think they need a "filter", really, or any sort of approval. From my understanding the entire purpose of the Spellcasters coven was the lack of regulation, allowing all members to participate fully regardless of how unqualified or, unfortunately, "fluffy" they are.
What I think would be more helpful is to have a system for removing obviously fake spells. I know there's editors an publishers and whatnot- maybe they can elect for spells to be removed, and either Pet himself or the mods or some sort of committee set up for this purpose can make the final call to remove them. I know it's probably more complex than that, especially if you consider Spellcasters a "real" coven (I don't, I personally view it as a means people use to post spells rather than a legitimate magical family, but then again I'm not one of them, so I can't really judge).