Serializing the broken state is quite tricky as this is driven by the simulation code
Since the breaking is hierarchical ( clusters can hold clusters for example ) you would need a way to capture the history of breaking and replay that when reloading the asset
In 5.1 this may be a bit simpler as we normally return the item index as part of the break event
Another option may be to set the simulatable attribute of the broken pieces to false ( see Geometry Collection Physics Proxy code ) and to hide them somehow that would force the geometry collection to not create a physics object for them
Hope this helps