The PostgresBuild 2021 slides of OrioleDB [1] (also linked in the GitHub project's readme) mention that there is a 1K LoC patch that adds features to the extension interface. I guess the patch is larger by now in 2023.

Slide 45 specifically lists:

* Extended table AM

* Custom toast handlers

* Custom row identifiers

* Custom error cleanup

* Recovery & checkpointer hooks

* Snapshot hooks

[1] https://www.slideshare.net/AlexanderKorotkov/solving-postgre...