At first I tried to use the bullet behaviour to create the effect of the loot popping up out of the smashed crate, but I couldnt get that working the way I wanted. The coins would bounce on the platforms, then just fall through.
In the end I added the physics behaviour to the loot (this is my first time experimenting with physics). It achieves the look I wanted, but I'm worried it might impact performance on mobile. There will only ever be 10-20 loot items on screen at once I guess, and they are inactive once they come to rest, so I hope that wont have too much impact? Regardless I'll have to test performance on some mobile devices soon... which means figuring out Phonegap I guess...
I have no idea if it's better than the physics behaviour, but I guess you could also do the same with the platform behaviour