@kristoffer.carlsson would be best qualified to answer, but I will try based on my interpretation of
- Add support for "package extensions" to code loading by KristofferC · Pull Request #47695 · JuliaLang/julia · GitHub .
- Code Loading · The Julia Language (dev version)
- 5. Creating Packages · Pkg.jl (dev version)
From what I can tell, the extension mechanism is primarily focused on extending methods, not necessarily introducing or exporting new types.
Based on Are extension packages importable? it might be possible to get a reference to the extension module itself.