Digging around in the code ... subsites already implement callbacks for state changes on objects (insert, update, etc) so while this isn't a general mechanism it is one to explore ...
This facility could be used for the add user case, for instance.
I think it only supports those for calls to
package_instantiate_object and it's ilk, and as far as I can tell
it's never been used (in fact I was considering posting about
removing it since it does a query on every object instantiate
which is empty but should almost certainly be aggressively cached).