Seems like there is room for a PostgreSQL-only ORM that specifically makes all these nice features available. Anyone heard of such a thing?

Most "frameworks"/ORM can provide database-specific features, and it's especially easy for field types. e.g. http://docs.sqlalchemy.org/en/rel_0_9/dialects/postgresql.ht... (data types local to SQL Alchemy's Postgres dialect) or https://github.com/djangonauts/django-hstore (hstore in Django)

There isn't really a need, though. SQLAlchemy exposes all of SQL, most unique things dialects offer and it makes it very easy to add types too.

Not exactly a PostgreSQL-only ORM

The excellent slick-pg library which extends the FRM library slick. https://github.com/tminglei/slick-pg

Not really an ORM but highly integrated with Postgres: https://github.com/ngs-doo/revenj

There are ORMs like SQLAlchemy (Python) and Sequel (Ruby) which try to expose as much as possible of the features of all supported databases. It is common for Sequel to add support for new PostgreSQL features while the new PostgreSQL version is still in beta testing.