There've been a few pull requests and issues for firing callbacks on embedded docs when the parent is saved. Right now, MONGOID-1059 is the most up-to-date. It's a pull request, though, so I'm creating an issue for this feature so that it can be tracked more easily.
As far as I can tell, MONGOID-1059 is solid, and the tests pass. However, my experience with Mongoid's callback implementation is limited. Thus, I'd appreciate durran or someone else with a lot of experience with Mongoid's callbacks to examine this pull request.