You could do it via soft-fork. You just need to add a rule that the transactions for the first 50% of the block has a POS proof.
POS N: validated by POS
POW N+1:
POS N+1: validated by POS
POW N+1:
You would need to add a modification to the protocol to propagate the POS proofs, but that isn't a massive deal.
Non-upgraded clients would just track the POW blocks, but each POW block would need a POS block proof to be accepted by other miners.
That is actually an interesting idea. The downside would be that it would effectively be a blocksize decrease as older nodes won't see POS blocks, only to see the relevant txns 20min later in the next POW block.
My personal opinion on the softfork idea is that if we are going increase blocksize sometime in the future, we would need a hardfork anyway. Increasing the blocksize has centralising effects and implementing this proposal together would do much to counteract and even improve decentralisation considerably.
One characteristic of this system is that hashing power is useless between the time a POW block is found and the next POS block is found.
Yes, this has been brought up, although id like to hear from a miner why this might be an issue if all miners are affected equally by this.