I'm going to close this issue, turns out the need for it in OGM has vanished. I took another look at the situation where it would have been useful, but I found another way which solely works with the postInstantiate() hook.

Specifically why won't a solution like a SessionFactoryObserver work

Yes, that should have worked as well. It would have been my alternative (in the current code we do the concerned logic repeatedly upon actual persister invocations), but I found a specific life-cycle hook more appalling. Of course we can implement that ourselves using an observer