Well, I don't know how to use sed, so I didn't go that route. I'd downloaded all the files and an export of the MySQL database before starting anything. Then I started using TexFinderX to search and replace all the crap at the beginnings of every php file here on my local copy. But the junk was still in there somewhere, and I was having no luck finding it. (Sed or TexFinderX or whatever tool you use only works if you know what you're looking for. I don't know what code was sticking that stupid rr.nu crap in there. I couldn't find it.)
So I just moved all the files on the server into a temp folder (much faster than deleting them with my ftp program) and uploaded a freshly downloaded version of Concrete5 4.2.2. I had to create a new database to install it into, then switch it back to the old database once it was installed. Oh, and I had to re-upload the theme I'd been using. Voila. Problem solved.
For now, anyway.