We did this with MotherDuck, and without introducing a new language. Every tenant has their own isolated storage and compute, so it’s trivial to grant internal users access to specific tenants as needed. DuckDB’s SQL dialect is mostly just Postgres’ with some nice ergonomic additions and a host of extra functionality.
This is explicitly not the problem they are trying to solve. In a single tenant database you don’t have to by definition worry about multi tenant databases