Figured this out and am posting the solution here just in case this weirdness affects anyone in the future.
About five days before this issue arose I had a catastrophic failure of my Visual Studio 2015 solution. Completely corrupted. Would not open. So I generated a clone from my last good commit on GitHub and continued working on my code.
This evening I attempted to add a radAboutBox to my WinForms project. I got this message:
Your project uses GAC referenced assemblies. Please upgrade your project.
When I attempted to use the Telerik Upgrade Wizard, I got this error message:
An error occurred while running the wizard.
Error executing custom action Telerik.WinControls.VSX.Actions.MultiProjectUpdateReferencesAction: System.IO.FileNotFoundException: Could not load file or assembly 'Telerik.WinControls.dll' or one of its dependencies.
Sure enough, my \lib\RCWF\2016.2.608.40 subdirectory (which contains the XML and DLL files for the WinForms Telerik controls) was missing about 50 files (the GitHub clone only restored a subset omitting very important ones including Telerik.WinControls.dll). I restored the missing files from a local backup and my radMessageBox displayed with a custom theme as expected via this code:
RadMessageBox.Show(this, "messagebox text", "title", MessageBoxButtons.OK, RadMessageIcon.Info);