Does it support the extension ecosystem? Or would extensions need to be rewritten as well?

It is theoretically possible to have a Rust port of Postgres support extensions. If you make all the relevant functions and structures ABI compatible with Postgres, extensions should work. The issue is the moment you're dealing with C pointers and C strings, pretty much all the code you have to write is unsafe.

Perhaps pure pgrx extensions would make sense as a first target?

They would need to be rewritten as there is no formal extension API. Extensions can call into almost any part of PostgreSQL.

They would need rewriting (a few are included)