Thanks Newt - by base64 do you mean a Data URI? Ashley mentions this here...
"whenever you save to a slot, also write a WebStorage key that indicates the slot has been saved to. You could add some other metadata like the player name or even a small screenshot of the game as a data URI"
...that's basically what I'm trying to do - I've got 12 slots, and I can save different game states in each slot with a small thumbnail - but when I load an earlier state, it reverts all the thumbnails to how they were at that point
What would be really cool is if you could change an objects "No-Save" state in code. Then I could set the blank slots to "No-Save" when saving, but set them to "Save" When they have a thumbnail in them. Then, when I load an earlier slot, all the filled slots will be left alone.