I started writing a Postgres client using asyncdispatch a while back, but never got around to finishing it: https://github.com/euantorano/postgres.nim
It currently supports basic queries, but doesn't yet support prepared statements (well, it can prepare a statement but not actually run it yet). The tests are a good way to see what's currently supported: https://github.com/euantorano/postgres.nim/blob/master/tests/postgres_async.nim
I do want to finish it off, I just need to find the time to do so...