Why would you want to pool anyway? PoS mining is a lot "easier" than PoW and PPC would be more secure if there are a lot of small miners than only a few pools.
In fact, pools for PoS mining would be almost the same as banks. As I understand decentralized currencies are much about bypassing big banks (apart from central banks obviously)
I want to pool because I don't want to have software running on my computer 24X7, even if doesn't use CPU. It's just more convenient.
Let's put it another way - I'm willing to loan my money to anyone who can promise:
1. To share at least 50% of the Stake reward with me.
2. To behave honestly and not try to "51% attack" the network.3
3. To implement the loan mechanism in a way such that I don't have to trust him and that I'll definitely get my money back.
I don't know if scripts can be used to achieve #3, or perhaps a specific protocol change in PPCoin is needed.