The approach I've taken is to implement my own application gateway using node-http-proxy.
I use npm workspaces to share common functionality between all the node parts of the application, including the api gateway / reverse proxy.
I configure the other services to trust a HTTP header that's injected in the api gateway (and explicitly deleted from incoming requests) that includes details about the user. For example, Apache Superset (and several other services) support the REMOTE_USER header.
Honestly, doing this in a high-level language like NodeJS with node-http-proxy is even simpler to do and easier to read / audit than what I've seen in Rego, plus I get to use all the common utilities for dynamic service access, database access, etc...
This borders on a "NIH" thing, but I think if you saw the actual implementation, it's even less complex than what I'm seeing from these examples. It took me a couple hours to throw together, it's easy to extend, and supports a multitude of authentication scenarios, including shared-session (which would be tough with Rego).