After writing my message, I recognized a scenerio you might be referring to Kevin.
If I understand correctly, you're not referring to set-membership in the general sense, where one could add and remove entries. General set-membership, in the context of eventual-consistency, requires timestamps. The timestamps distinguish between the two values "present" and "not-present". (not-present being represented by timestamped tombstones in the case of deletion/removal).
So I suppose you're referring to "additive-only set membership", where there is no need to distinguish between two different states (such as present or not present in a set), because items can only be added, never changed or removed. If entries are not allowed to be deleted or modified, then cassandra-style eventual consistency replication could occur without any timestamp, because you're simply replicating the existence of keys to all replicas.
To me this seems a particularly narrow use-case. Any inadvertant write (even one from a bug or data-corruption), would require very frustrating manual intervention to remove. (you'd have to manually shutdown all nodes, manually purge bad values out of the dataset, then bring the nodes back online) I'm not a cassandra developer, but this seems like a path which is very specialized and not very in-line with Cassandra's design.
You might have better luck with a distributed store that is not based on timestamp eventual consistency. I don't know if you can explicitly turn off timestamps in HBase, but AFAIK the client is allowed to supply them, so you can just supply zero and they should be compressed out quite well.