Someone pointed out that other people might like to learn how to do it, too. Short version: watch this video, which I finally tracked down after five months of looking (go, general research skills!).
Longer version: You’ll need a camera with a memory card, the Apple iPad Camera Connection Kit (MC531ZM/A), and the iPad app Splice (I spent the four bucks for it). What you do is transfer the video you want to modify to the Photo app, open up Splice, find the video that you want to work on, and export it over to Photo’s Camera Roll (High Quality setting). Then go to iMovie, and hey presto! It’s visible and you can work on it.
My only caveat: I don’t know how well it does, or how fast it is, with longer videos. But it works. And this Youtube fellow oktopus74 is a Great Man for putting the original video up.