I was unable to come up with any suitable solution that also preserved PoS checkpointing functionality - IMO it just can't be done (the right way) with current hybrid PoW/PoS design. In PoS-only system - no problem. But with hybrid chain it's just one hell of a mess.
This should be weighed carefully. PoW is a bit hard to rely on long term due to not knowing how technology is going to affect it, not knowing if someone will plug in massive hashpower out of the blue someday, not knowing how future energy costs will factor in, etc.