I really want to love foreign data wrappers for Postgres and this seems like a big improvement over existing Python library, but the lack of support for them in managed databases services makes them a non-starter for so many use-cases.

Because RDS, for example, will only support the foreign data wrapper for reading from another "Postgres", what we really need is a server that supports the Postgres wire protocol (easier said than done) and you implement your drivers as a handler to that server.