Depending upon your "build" vs. "buy" perspective... if you're interested in building your own solution, check out this Salesforce project:
If you're interested in "buying" instead, we are one of a handful of vendors who provide OData capabilities for various sources without any modification or changes to the underlying data source. We can definitely OData-enable Postgres in RDS. We've used this to integrate Oracle and SQL Server data on RDS into Salesforce with Salesforce Connect. We've also successfully tested integrating Postgres running on Compose.io, which is now part of IBM.
We offer a SaaS service, Datadirect Cloud:
https://www.progress.com/cloud-data-integration. You can trial this at no charge, by simply filling out a form. Fully functional, 30 days.
We also offer a product that you can host yourself on your choice of infrastructure. (It requires an Intel/linux/java system.) It's called Hybrid Data Pipeline. https://www.progress.com/hybrid-data-integration. Similar functionality - dynamically add OData capabilities to selected data sources.
You can trial this, too, by filling out a form and downloading the installer, but it would involve having a system to install it on. A little more work. FYI - it seems that Amazon's new Lightsail virtual server offering is an easy way to trial Hybrid Data Pipeline. You have to use the largest system, though.