Much like POW mining pools, I bet there would be some sort of way to do a CLAM POS staking lottery pool....
The trouble with this is that a pool "re"-centralizes what should be a decentralized operation, and it's worse than PoW. If an operator of a PoW pool goes rogue, miners may lose funds, but they stop sending shares; if the op goes rogue in a PoS pool (eg by disabling withdrawals) he retains a tremendous amount of PoS power, in perpetuity.
The other issue with a PoS pool is that you'd have to keep a large amount of your funds with a third party. With a PoW pool you can basically withdraw as fast as you mine.
And finally, if the op was able to modify the code the pool could choose to stake only when it's worth it. With such a large weight (balance) behind it there is a good chance the pool would win the majority of the lotto blocks, therefore, others have no choice but to join the pool, making the pool even stronger and more centralized, compounding the above issues...
tl;dr a rogue PoS pool op can 51% the network or run off with your funds