What is the most correct / typical / popular namespace to associate an organization name, logo, and maybe website metadata with an RSS 2 (or Atom) feed entry? Can we just use https://schema.org/Organization for that? Organization can be understood as a creator of the entry or original creator (in reposting case) or maybe some more involved relation approximated with the association.
The best representation I've seen are mixtures of Microformats, Schema.org, and Dublin Core. Essentially, utilize the semantics that are best for your situation; it sounds
rel="author" is what you are looking for in differentiating blog posts. Indie Web Camp's feed and A List Apart's feed are great examples of metadata in feeds.
Running them through pin13's validator exposes their semantics; not surprisingly, nothing renders in Google's "Structured" Data Testing Tool.