Hm, for content fragments I don't know ... You can only place widgets in CFs and not full-blown components which you could implement as Sling Models and add proper communication to an external endpoint.
But if you use Experience fragments that is possible, as they are backed by "real components" 🙂
Maybe it's also possible to implement CF with such data but it's definitly not that easy, and probably needs to rely on implementation details. Nevertheless it would be nice to have an extension point for that without the need to import data in the repo (which you can do for CFs very easy, but sometimes that's not what you want).
regards,
Jörg