From what I understand, that should be possible with Fly.io, using the fly-replay header to denote requests that need to be run against the primary database. I'm not sure if that's still their recommended approach, though.

EDIT: https://fly.io/blog/globally-distributed-postgres/ talks about how this can work. The approach it suggests for implementing this is to try running all database queries locally; if there's an error from the local database being a read-only replica, add the fly-replay header pointing to the primary region and return an HTTP 409, and Fly's proxy will rerun the request in the specified region. There's also some commentary on handling consistency issues to at least get read-your-own-writes.