You might be able to argue that Postgres' foreign data is a different form of pluggable storage engine. E.g. you can even use a CSV file as a backing store for a table with file_fdw.

https://www.postgresql.org/docs/current/ddl-foreign-data.htm...

https://dbaclass.com/article/how-to-access-csv-files-on-file...

Someone should do an experience report on adding the same storage system to both databases.

What you say looks powerful but Facebook added their MyRocks to MySQL.