No offense taken! Yes, I'd selected HKCU. After thinking about it for a while longer, I tried the very test you suggested and came to the same conclusion that you did, so I opened Regedit and navigated to "HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Excel." As we both suspected, "Recent Files" didn't exist--after creating it, I was able to set the Registry exception in CCleaner.
Strangely, this had happened after I had intentionally opened a file in Excel, closed Excel, then reopened Excel to confirm that the file was listed in the "File" menu (which it was). Even after all that, the "Recent Files" key didn't exist. Puzzling.
Any way, the story has a happy ending--in my latest test, my Excel recent files were preserved! Thanks for the help, Disk4mat.