Now that it’s out there I can show you how I did it.
The intermediate app
The trick is this — the bookmarklet calls an app running in a web page on fargo.io.
This is where the app is: http://fargo.io/misc/bookmarklet/.
Here’s a screen shot of the source of the app.
Because that app comes from the same domain as the Fargo app, when they’re running on your machine they share a local storage namespace.
The app writes the query string to localStorage.fargoSearchString, and it quits.
Once a second, Fargo looks for localStorage.fargoSearchString. When it shows up, it copies it, deletes it, and then parses it, and adds the link to the designated linkblog outline.
Why it works
We just communicated with a running app without having to relaunch it. Which of course with a major piece of software like Fargo is a fairly big deal — far too much work to do for a single link. But the intermediate app is just a few lines of text. No big deal loading it just to send a message to Fargo.