I guess technically you could as bollu says rewind yourself by calculating all the physics in reverse, but if you go that far you could as well recode chipmunk ^^
dietweb solution seems the most likely from what you descibe, writting all the information of the shapes and bodies then when you reopen your app you'll have all you data.but then doing such a write would impact on the perf of your game even if you did it once evey second.
What I would like to bring up is the game design "flaw" I think originated this issue. if your game is such fast paced game as you put it, even if you managed to save 2 sec before in time, the time the user react s to the app having reopened he will have died again. I mean if so "little" time is important in the game, the user might get suprised being plunged in the fast paced so suddenly lol (I know it happens to me so often when playing I get back in the game without paying too much attention and bam... dead).
I find it strange this callback issue, I was thinking, ok there is a callback when the application closes, but is there a way to listen at the keys of the devices like volume up and down etc. if there is you could tirgger your own callback for saving
Hope this helps