Finally, SMMSTORE: it exists, it helps where it is supported to persist UEFI variables, but as I understand it, actual support for devices is rather limited.
SMMSTORE_V2 is fully working with the current coreboot default option for Tianocore (UEFIPAYLOAD), and is selected by default.
We are still using v1, but I want to switch us to v2 on our next edk2 update. My only concern is migrating the store from the v1 to the v2 format (or rather, the format edk2 writes when using v2). I was planning on doing it in our firmware-smmstore UEFI app, but if anyone else would need this functionality it would probably be better to add it somewhere in BlSMMStore.
I will rebase our changes on Matt's branch to see how many our fork still needs, probably some time next week.