Sqlite has a concept called virtual tables, which sit somewhere between "pluggable storage" and postgres foreign-data-wappers. They are fairly easy to implement and can be incrementally improved by providing more sophisticated hints to the planner. I think the folks at tarantool have done some work hooking deeply into sqlite's pager/wal, presumably others (the folks who connected sqlite to lmdb?). And of course Oracle, who ripped out the sqlite btree and replaced it with BerkeleyDB.

I think sqlite4 (defunct) was going to have pluggable storage, but they ended up scrapping it. Presumably over performance concerns, but please correct me if I'm wrong.

Anyway, interested to see how this feature is adopted and to learn more.