Google Reader API - How do you share by article url as oppose to item id?

I've done a decent amount of research on the unofficial reader api and sifted through other questions, but none of them quite have what I want. How to share an article if you know the article id is well documented, but I want to know how to share an article if you don't know the id (i.e. how to transform url -> id or how to share directly using the url as a parameter).

I imagine it is possible, because the functionality pretty much mimics the behavior of the awesome "Note in Reader" bookmarklet they have.

Thanks!