As far as I can see now, using difficulty as score for POW blocks, and let POS blocks inherit that score from the previous POW block is a good enough solution for the problem on hand. Whether one POS block following another POS block is explicitly ruled out or not, I think it should be ok either way.

The Novacoin solution of encouraging a hybrid chain only make sense because they have 1:1 POW:POS blocks ratio. YACoin is 10:1 with POW:POS. You'll need to check pretty deep into the past to encourage a 10:1 ratio. The score consistency problem I mentioned above could lurk in such schemes. Of course, we can also talk about the POW:POS ratio if people want to change it.