The pull request introducing ForeignReferences was merged this week 
It is a something to build on. But it may be too early to introduce it as a new file format.
Iām inclined to keep it an implementation detail for the software implementing the Friendly Forge Format. In the case of Gitea it means providing a factory that returns an object with an interface able to store and retrieve a foreign reference. And the Go reference implementation of the Friendly Forge Format provides a SQLite based alternative for test purposes.