Good question 
Adding / removing “poor validators” would require regeneration of the 'Decentralized Validator" ECDSA key.
If Casper supported validator key rotation (which is does not support now, but probably will), you could add/remove poor validators at anytime, so then you could tokenize membership in the validator contract.
In the current implementation you could also tokenize by requiring the validators to rotate the ECDSA key say each month, the “Decentralized Validator” would have to withdraw and re-join at the end of each month.
I would be happy to put 15 ETH for an experiment like this, if there are 99 more people interested. This could be a way for security researchers to jointly run a validator to experiment with Casper once it is live. Note that this is very different from running a pool because all decisions are made by joint votes.