For my own purposes I have also added a little helper to magit-rockstar:
magit-branch-pull-request. But note that everything in that library is provided as-is (i.e. instead of blogging about snippets, I stuff them into this library). From the library commentary:
Also included are tools that are either only useful for people working on Magit itself and/or that aren't ready to be added to Magit yet. These tools might change at any time, without prior notice or way to appeal. This is a staging ground. It's okay if things ain't perfect, or if they only do what I currently need but not what you (or I) think they should (eventually) be doing instead.
Of course magit-gh-pulls is another, more complete, alternative, but I intend to eventually replace that with something even more feature-full directly in Magit. Until I get there,
magit-branch-pull-requests provides the basic feature that I need. While
magit-gh-pulls does a few things in ways that I don't think are optimal. (Most importantly, it brings in dependencies I would like to avoid (in particular gh.el which has been accumulating issues)).