If you're interested in this sort of thing, Doorkeeper[1] is a robust, open source OAuth 2 provider that's been around for about 5 years. We use it as a standalone app, and have many other node.js apps that sign in using it.
Doorkeeper is closer to a full-package with customizable features, including a basic frontend. I'm not too familiar with hydra, but it seems Doorkeeper is best when you want to get the full OAuth app & user interface running (and customize later), whereas Hydra is best when you want to get a quick OAuth API app and build your own frontend. Would you say this is accurate?
Doesn't the nature of an OAuth server imply that it can be added to existing infrastructures? Or is there an issue you foresee with non-Hydra libraries?
[1] https://github.com/doorkeeper-gem/doorkeeper