One solution for this which has been proposed by several people, which is quite different from the solution proposed in this pre-RFC, is adding namespacing purely on the crates.io side (or an alternative registry's side) in a way that doesn't require any changes to the Rust module system or even its knowledge/awareness of namespaces (i.e. a registry-side-only change).
This could be accomplished by letting someone claim the foobarbaz-* / foobarbaz_*-prefixed namespace if and only if there are no published crates with foobarbaz-* / foobarbaz_* prefixes already and the owner requesting the namespace controls the foobarbaz crate.
This would be a codification of what people already do in order to have a pseudo-namespace for their crates.