Before implementing Federation we have been using a testing process with Review Applications (RA) where our merge requests are able to deploy their own standalone containerized app and backing datastore (if necessary) to test against existing resources before merging with the main branch and ultimately deploying to a “real” environment. See Netlify Deploy Previews, for an example of this. Federation comes with additional complexity here since we would need to also deploy a gateway either in a) non-managed federation mode using the RA subgraph URL b) managed-federation mode with a new variant which includes the RA subgraph URL. At least those are what I see as the possible options.
Now I know this isn’t technically a Federation specific issue since you would need to address this with any gateway-like solution, but I’m curious if anyone out there has solved the problem elegantly.