GRDDL
DanBri pointed me to a specification named GRDDL. It looks like exactly what I was looking for last August. Any XML file is one namespace declaration, one attribute definition, and one XSLT file away from RDF.
Oddly, it defines a completely different mechanism for XHTML. I'm not sure what I think about that.
Definately something to watch, and probably something to include in a best practices guide to Atom.