The Harmony cart will be able to do save states with some existing or future games. I got a proof of concept of this working recently, and tried it in Solaris and a few other games.
However, note that the Harmony is a 2600 cart. You aren't going to be playing 7800 Midnight Mutants on it with or without save states.
That's cool, It's good to know that this is possible. Please keep me posted on this when it is ready to sell.