I think the lottery was poorly thought out, but not intentionally exploitable. I didn't hear that Rat4 was ever thought to be a major holder of CLAMs. It's possible though.
Not sure if this would considered exploitable, but you could have gained quite an advantage by only staking selectively. Staking disabled by default; call GetProofOfStakeReward right after a new block arrives, then enable staking ONLY if the reward for the next block is sufficient. In addition to not wasting stakes on 0.1 reward blocks, by ignoring those the overall weight when it does attempt to stake would be greater.